Emulation suspension mode with frame controlled resource access

ABSTRACT

Emulation and debug circuitry is provided that can be incorporated into a variety of digital systems. A stop mode of operation is provided in which an associated processor stops processing instructions in response to a debug event. A real-time mode of operation is provided in which the processor stops processing background instructions in response to a debug event, but in which high priority interrupts are still processed. Interrupts are classified and processed accordingly when the processor is stopped by a debug event. While suspended for a debug event, a frame counter keeps track of interrupt debug state if multiple interrupts occur. While running or suspended, the emulation circuitry can jam an instruction into the instruction register of the processor to cause processor resources to be read or written on behalf of the emulation circuitry. Read/write transactions are qualified by an expected frame count to maintain correspondence between test host software and multiple debug/interrupt events. Architecture and instruction set are optimized for low power consumption and high efficiency execution of DSP algorithms, such as for wireless telephones, as well as pure control tasks.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 USC §119(e)(1) of ProvisionalApplication No. 60/120,809, filed Feb. 19, 1999.

This application is related to co-assigned application S.No. 60/120,809;Ser. Nos. 09/481,852; 09/483,568; 09/489,902; S.Nos. 60/120,683;60/120,960; 60/120,791; 60/120,810; 60/120,667; and Ser. No. 09/154,385;filed contemporaneously herewith and incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to digital microprocessors, and moreparticularly to emulating and debugging digital microprocessors.

BACKGROUND OF THE INVENTION

As the technology for manufacturing integrated circuits advances, moreand more logic functions may be included in a single integrated circuitdevice. Modern integrated circuit (IC) devices include large numbers ofgates on a single semiconductor chip, with these gates interconnected soas to perform multiple and complex functions, such as, for example,those in a general-purpose microprocessor. The manufacture of suchcircuits incorporating such Very Large Scale Integration (VLSI) requiresthat the fabrication of the circuit be error free, as some manufacturingdefects may prevent it from performing all of the functions that it isdesigned to perform. This requires verification of the design of thecircuit and also various types of electrical testing after manufacture.

In conjunction with the stuck-fault modeling and associated testgeneration, other circuitry may be included in the VLSI circuitspecifically designed to improving its testability. One type of testcircuitry is a scan path in the logic circuit. A scan path consists of achain of synchronously clocked master/slave latches (or registers), eachof which is connected to a particular node in the logic circuit. Theselatches can be loaded with a serial data stream (“scan in”) presettingthe logic circuit nodes to a predetermined state. The logic circuit thencan be exercised in normal fashion, with the result of the operation (ateach of the nodes having a scan latch) stored in its respective latch.By serially unloading the contents of the latches (“scan out”), theresult of the particular test operation at the associated nodes is readout and may be analyzed for improper node operation. Repetition of thisoperation with a number of different data patterns effectively tests allnecessary combinations of the logic circuit, but with a reduced testtime and cost compared to separately testing each active component orcell and all their possible interactions. Scan paths permit circuitinitialization by directly writing to the latches (or registers) anddirectly observing the contents of the latches (or registers). Usingscan paths helps to reduce the quantity of test vectors compared totraditional “functional mode” approaches. Techniques for scanning suchdata are discussed by E. J. McCluskey in A Survey of Design forTestability Scan Techniques, VLSI Design (Vol. 5, No. 12, pp. 38-61,December 1984).

Another solution is the test access port and boundary-scan architecturedefined by the IEEE 1149.1 standard, a so-called JTAG test port. IEEE1149.1 is primarily intended as a system test solution. The IEEE 1149.1standard requires a minimum of four package pins to be dedicated to thetest function. The IEEE 1149.1 standard requires boundary scan cells foreach I/O buffer, which adds data delay to all normal operation functionpins as well as silicon overhead. Although it has “hooks” forcontrolling some internal testability schemes, it is not optimized forchip-level testing. IEEE 1149.1 does not explicitly support testing ofinternal DC parametrics.

Software breakpoints (SWBP) provide another mechanism to allow the debugof microprocessor code and to evaluate performance. A SWBP is typicallyaccomplished through opcode replacement, provided the program resides ina writable memory module which allows the opcode at the stop point to bereplaced in memory with the software breakpoint opcode. In mostmachines, when a SWBP opcode reaches the first execute stage of aninstruction execution pipeline, it causes the pipeline to stop advancingor trap to an interrupt service routine, and set a debug status bitindicating the pipeline has stopped or trapped. In processors classifiedas protected pipelines, instructions fetched into the pipeline after theSWBP are not executed. Instructions that are already in the pipeline areallowed to complete. To restart execution the pipeline can be clearedand then restarted by simply refetching the opcode at the SWBP memoryaddress after the opcode is replaced in memory with the original opcode.

Testing and debugging such a complex pipeline is difficult, even whenthe techniques described in the preceding paragraphs are used. Aspectsof the present invention provide improved methods and apparatus forchip-level testing, as well as system-level debugging.

SUMMARY OF THE INVENTION

Particular and preferred aspects of the invention are set out in theaccompanying independent and dependent claims. Combinations of featuresfrom the dependent claims may be combined with features of theindependent claims as appropriate and not merely as explicitly set outin the claims. The present invention is directed to improving theperformance of processors, such as for example, but not exclusively,digital signal processors.

A digital system is provided with a processor, wherein the processor isoperable to execute a sequence of instructions obtained from aninstruction bus connected to an instruction memory circuit. Theprocessor has system resources including registers and at least onememory circuit. The processor has condition circuitry for storing atleast one condition. There is test port circuitry for receiving testcommands from a remote test host. Emulation circuitry for debug eventsis connected to the test port circuitry, operable to cause the processorto enter a debug suspend state in response to a debug event and to leavethe debug suspend state to resume execution of the sequence ofinstruction in response to a command received by the test portcircuitry, wherein execution of the sequence of instructions ceaseswhile the processor is in the debug suspend state. The emulationcircuitry is operable to access the system resources in response to anaccess command received by the test port circuitry if a first conditionof the at least one condition is in a first state; however, theemulation circuitry is blocked from accessing the system resources inresponse to an access command received by the test port circuitry if thefirst condition is in a second state.

According to another aspect of the present invention, the conditioncircuitry comprises debug enable mask bit (DBGM) circuitry operable tostore either the first state or the second state in response to aninstruction executed by the processor from the sequence of instructions.

According to another aspect of the present invention, the conditioncircuitry comprises high priority interrupt (HPI) circuitry operable tostore the second state in response to the first interrupt and furtheroperable to store the first state at the completion of the first ISR.

According to another aspect of the present invention, the processor hasdebug frame counter (DFC) circuitry operable to store a debug framenumber corresponding to a first debug event, wherein the test portcircuitry is operable to communicate the frame count number to theexternal test host. There is debug frame register (DFR) circuitry,operable to store an expected frame count number in response to acommand received by the test port circuitry. A first access commandreceived by the test port circuitry is not performed if the frame countnumber is different from the expected frame count number.

According to another aspect of the present invention, there is memoryunit cycle type (MCT) circuitry connected to the emulation circuitryoperable to specify a selected access type corresponding to the firstcondition. The emulation circuitry is operable to access the systemresources in response to an access command for the selected access typereceived by the test port circuitry even if the first condition of theat least one condition is in the second state.

According to another aspect of the present invention, a method isprovided for operating a digital system having a processor with systemresources including registers and at least one memory circuit,comprising the steps of storing at least one condition within thedigital system; receiving an access command from a remote test host;accessing the system resources in response to the access command if afirst condition of the at least one condition is in a first state; andblocking access to the system resources in response to the accesscommand if the first condition is in a second state.

BRIEF DESCRIPTION OF THE DRAWINGS

Particular embodiments in accordance with the invention will now bedescribed, by way of example only, and with reference to theaccompanying drawings in which like reference signs are used to denotelike parts and in which the Figures relate to the processor of FIG. 1,unless otherwise stated, and in which:

FIG. 1 is a block diagram illustrating an emulation system connected toa target device which embodies aspects of the present invention;

FIG. 2 is a block diagram illustrating an emulation system connected toseveral modules within the target device of FIG. 1;

FIG. 3 is a state diagram illustrating real-time mode execution states;

FIG. 4 is a state diagram illustrating stop mode execution states;

FIG. 5 is a state diagram illustrating real-time mode execution states;

FIG. 6 is a state diagram illustrating the combined state machines ofFIG. 3 through FIG. 5;

FIG. 7 is a block diagram of the target device of FIG. 1;

FIG. 8 is a representation of the interrupt flag register of the targetdevice;

FIG. 9 is a representation of the interrupt enable register of thetarget device;

FIG. 10 is a representation of the debug interrupt enable register ofthe target device;

FIG. 11 is a flow chart illustrating processing of a maskable interruptrequest;

FIG. 12 is a flow chart of interrupt processing while in the executestate;

FIG. 13 is a flow chart of interrupt processing while in the debugsuspend state;

FIG. 14 is a flow chart of steps performed within a high priorityinterrupt service routine when a high priority interrupt occurs duringthe debug state;

FIG. 15 is a block diagram of an embodiment of emulation circuitryaccording to the present invention;

FIG. 16 is a block diagram of the debug core of FIG. 15;

FIG. 17 a more detailed block diagram of the emulation circuitry of FIG.15;

FIG. 18 is a schematic representation of an integrated circuitincorporating the processor; and

FIG. 19 is a schematic representation of a telecommunications deviceincorporating the processor of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Although the invention finds particular application to Digital SignalProcessors (DSPs), implemented, for example, in an Application SpecificIntegrated Circuit (ASIC), it also finds application to other forms ofprocessors. FIG. 1 is a block diagram of a digital system 10 which hasan embodiment of the present invention, and will be described in moredetail in later paragraphs. In the interest of clarity, figures hereinonly show those portions of digital system 10 that are relevant to anunderstanding of an embodiment of the present invention. Details ofgeneral construction for DSPs are well known, and may be found readilyelsewhere. For example, U.S. Pat. No. 5,072,418 issued to FrederickBoutaud, et al, describes a DSP in detail and is incorporated herein byreference. U.S. Pat. No. 5,329,471 issued to Gary Swoboda, et al,describes in detail how to test and emulate a DSP and is incorporatedherein by reference. U.S. Pat. No. 5,828,824 issued to Gary Swobodadescribes improvements to an emulation system using extended operatingmodes and is incorporated herein by reference. An embodiment of thepresent invention is an improvement on the structure of the IEEE1149.1—1990 Standard Test Access Port and Boundary Scan Architecture,which is incorporated herein by reference. Terms and concepts relatingto IEEE 1149.1 which are used herein are explained fully in this IEEEstandard. Details of portions of digital systems relevant to anembodiment of the present invention are explained in sufficient detailhereinbelow, so as to enable one of ordinary skill in the microprocessorart to make and use the invention.

FIG. 1 is a block diagram illustrating an emulation/debug system 14connected to a target device 10 which embodies aspects of the presentinvention. Test host 13 allows high-level debugging actions to beperformed by a user. Debug system 14 is connected to a low-levelinterface 11 supported by target device 10.

Debug Host 13 is a computer, typically a PC, running the user-interfacefor a target processor debugger. The debug host allows the user to issuehigh level commands such as “set breakpoint at function main( )” or“examine the contents of memory from 0x0 to 0x100”. The source code forthe debugger is available from Texas Instruments Incorporated as anEmulator Porting Kit (EPK), for example.

Scan Controller board 12 is attached to the debug host and is connectedto the target device via JTAG interface 11. The Scan Controller performsJTAG state sequences sent to it by the Debug Host. Scan controller 12can be XDS-510, XDS-510PP or XDS-510WS available from Texas InstrumentsIncorporated, for example.

Target device 10 contains hardware extensions for advanced debuggingfeatures. These assist in the user's development of the applicationsystem (software and the hardware). Since these capabilities are part ofcore processor 15, they are available on any device which includes coreprocessor 15, utilizing only the JTAG interface with extended operatingmode enhancements, as described in U.S. Pat. No. 5,828,824. They providesimple, inexpensive, and speed independent access to core 15 forsophisticated debugging and economical system development, withoutrequiring the costly cabling and access to processor pins required bytraditional emulator systems or intruding on system resources. Theon-chip development interface provides:

Non-intrusive access to internal and external memory.

Minimally-intrusive access to CPU and peripheral registers.

Control of the execution of background code while continuing to servicereal-time interrupts.

Break on a software breakpoint instruction (instruction replacement).

Break on a specified program or data access without requiringinstruction replacement (accomplished using bus comparators).

Break on external attention-request from debug host or additionalhardware (XDS-524 or logic analyzer or other processor).

Break after the execution of a single instruction (single-stepping)

Control over the execution of code from device power-up.

Non-intrusive determination of device status.

Detection of a system reset, emulation/test-logic reset, or power-downoccurrence.

Detecting the absence of a system clock or memory-ready signal.

Checking if global interrupts are enabled.

Determining why debug accesses might be blocked.

Rapid transfer of memory contents between the device and a host (datalogging).

Two 40-bit Parallel Signature Analysis registers (PSA) to verify properCPU operation at speed.

A 40-bit cycle counter for performance benchmarking. With a 100 MHzcycle clock, this can benchmark actions up to 3 hours in duration.

Ability to add more debug capabilities through ASIC macros built fromgate-array cells.

In an alternative embodiment, to reduce the silicon cost of emulationsupport hardware, many of the capabilities above can share the sameresources such that they could not be performed in parallel.

The example system in FIG. 2 shows the system connectivity necessary fordebug with multiple CPUs in multiple devices. This diagram omits signalbuffering and other electrical considerations necessary to create afunctional system. In this example, module 102 a and module 102 bcontain CPU cores while module 103 does not. The three modules share aparallel connection to signals nTRST, TCK, and TMS. The scan path beginsat the connector which provide JTAG interface 11 with signal TDI, entersmodule 103, passes through module 102 b, and exits module 102 a, endingas TDO back at the connector. Connections between module 1 and module 0nET1 and nET0 create trigger channels one and zero.

Scan Controller 12 supplies VDD (Power), GND (Ground), and TCK to theconnector. Scan controller 12 accommodates the system generation of(SYSTCK) or scan controller generation (TCKO) of TCK.

The multiprocessing debug environment requires a multi-tasking operatingsystem on Host 13. Using the multi-tasking OS framework allows multiplesingle processor debuggers to be spawned as separate tasks by a globaldebugger shell. This creates an environment that allows the user tomanipulate each processor individually via the single processordebuggers in individual windows or control processors globally via theglobal execution control directives which are part of the globaldebugger shell, as described in U.S. Pat. No. 5,828,824.

The following definitions will help the reader to understand theinformation in the rest of this application:

Background code: The body of code that can be halted during debuggingbecause it is not time-critical.

Foreground code: The code of time-critical interrupt service routines,which are executed even when background code is halted.

Debug-suspend state: The state in which the device does not executebackground code.

Time-critical interrupt: An interrupt that must be serviced even whenbackground code is halted. For example, a time-critical interrupt mightservice a motor controller or a high-speed timer.

Debug event: An action, such as the decoding of a software breakpointinstruction, the occurrence of an analysis breakpoint/watchpoint, or arequest from a host processor that can result in special debug behavior,such as halting the device or pulsing one of the signals EMU0 or EMU1.

Break event: A debug event that causes the device to enter thedebug-halt state.

A user of emulation system 14 has the option of selecting two forms ofcontrol over target processor 10's execution of code: stop mode andreal-time mode. Stop mode halts execution of all code whereas real-timemode allows selected interrupt service routines (ISRs) to be performedwhile execution of background code is halted. Background code refers tothe main body of code which is generally not as time-critical as theinterrupt routines which service motor controls or high-speed timers,for example. Real-time mode provides for the debug of code thatinteracts with interrupts that cannot be disabled. Stop mode providescomplete control of program execution, allowing for the disabling allinterrupts (including those which are non-maskable) and reset. Bothexecution modes can suspend program execution at debug events, such as:

software breakpoint instructions

specified program or data accesses (i.e. analysis breakpoints orwatchpoints)

when test host 13 or external hardware requests it

Real-time Debug Mode

Real-time debug allows for the suspension of background programexecution at break events while continuing to service time-criticalinterrupts (also referred to as foreground code). Program execution canactually be suspended in multiple locations—under certain circumstancesit is advantageous to break within one time critical interrupt serviceroutine while still allowing others to be serviced.

The suspension of execution is similar to the execution of an IDLEinstruction. An instruction pipeline within target device 10 flushes asinstruction decode stops and all current pipeline activity completes.Interrupts can restart execution, but after the interrupt serviceroutine is complete, the device returns to the suspended state (thisdiffers from IDLE—after servicing an interrupt the device does nottypically return to the IDLE state). When suspended, a debug interruptenable register (DBGIER) is used in addition to a standard interruptenable register (IER) to qualify pending interrupts. Interrupts must beenabled by both masks to interrupt the CPU when it is suspended,however, a global interrupt enable flag (INTM) is ignored. Suspendingexecution will add only one cycle to interrupt latency.

The DBGIER register serves to indicate what interrupts aretime-critical. When a time-critical hardware interrupt is taken, a highpriority interrupt active bit (HPI) in debug status register (DBGSTAT)is enabled (it's previous value is saved on the stack in the status wordDBGSTAT). HPI will be cleared when the last time-critical interruptservice routine completes and it's previous DBGSTAT is popped off thestack. HPI is used to indicate whether certain break events should beperformed. Note that HPI will be set whenever an interrupt enabled inthe DBGIER register is taken—not just when execution was suspended. Itwill not be set by an INTR instruction which corresponds to an interruptset in DBGIER.

Stop Mode Execution Control

Stop mode causes break events to suspend program execution at the nextinterrupt boundary, which is generally identical to the next instructionboundary. When execution is suspended, all interrupts (including resetand non-maskable interrupts) are ignored until the CPU receives a debugrun directive. Stop mode can be thought of as similar to real-time mode,except that no interrupts are considered time-critical (i.e., all codeis background code). Should an interrupt occur simultaneously with adebug event, the debug event has priority. However, once the processingof an interrupt has begun, target device 10 can not process a debugevent until the first instruction boundary following one of theseevents.

FIG. 3 is a state diagram illustrating real-time mode execution states.There are three or more execution real-time execution states. Thesestates are implemented by an Execute State Machine (ESM) withinemulation circuitry of core processor 15. For this embodiment, there arethree states defined: Execute (EXE) 300, Debug Suspend (DSUSP) 310, andInterrupt During Debug Suspend (IDS) 320. EXE state 300 represents codebeing executed on target device 10 with no outstanding stop conditions.

Two of these states, execute 300 and debug suspend 310, are also usedwhen in stop mode. IDS 320 cannot be entered from DSUSP state 310 duringstop mode debug.

FIG. 4 illustrates the relationship among two states that are includedwithin EXE state 300 a: single step state 301 and run state 302. Stopmode causes break events, such as software breakpoints and analysiswatchpoints, to suspend program execution at the next interrupt boundary(which is usually identical to the next instruction boundary). Whenexecution is suspended, all interrupts (including NMI and RS) areignored until the CPU receives a directive to run code again. In stopmode, the CPU can operate in the following execution states:

Debug-halt state 310 a. This state is entered through a break event,such as the decoding of a software breakpoint instruction or theoccurrence of an analysis breakpoint/watchpoint. This state can also beentered by a request from the host processor. In the stop modedebug-halt state, the CPU is halted. A user can place the device intoone of the other two states by giving the appropriate command to thedebugger.

The CPU cannot service any interrupts, including NMI and RS (reset).When multiple instances of the same interrupt occurs without the firstinstance being serviced, the later instances are lost.

Single-instruction state 301. This state is entered when a user tellsthe debugger to execute a single instruction by using a RUN 1 command ora STEP 1 command. The CPU executes the single instruction pointed to bythe PC and then returns to the debug-halt state (it executes from oneinterrupt boundary to the next). The CPU is only in thesingle-instruction state until that single instruction is done.

If an interrupt occurs in this state, the command used to enter thisstate determines whether that interrupt can be serviced. If a RUN 1command was used, the CPU can service the interrupt. If a STEP 1 commandwas used, the CPU cannot, even if the interrupt is NMI or RS.

Run state 302. This state is entered when a user uses a run command fromthe debugger interface. The CPU executes instructions until a debuggercommand or a debug event returns the CPU to the debug-halt state.

The CPU can service all interrupts in this state. When an interruptoccurs simultaneously with a debug event, the debug event has priority;however, if interrupt processing began before the debug event occurred,the debug event cannot be processed until the interrupt service routinebegins.

Notice that the processor cannot pass directly between thesingle-instruction and run states. Notice also that the CPU can beobserved only in the debug-halt state. In practical terms, this meansthe contents of CPU registers and memory are not updated in the debuggerdisplay in the single-instruction state or the run state. Maskableinterrupts occurring in any state are latched in the interrupt flagregister (IFR).

FIG. 5 illustrates the relationship among two states that are includedwithin EXE state 300 b: single step state 303 and run state 304.Real-time mode provides for the debugging of code that interacts withinterrupts that must not be disabled. Real-time mode allows a user tosuspend background code at break events while continuing to executetime-critical interrupt service routines (also referred to as foregroundcode). In real-time mode, the CPU can operate in the following executionstates.

Debug-halt state 310 b. This state is entered through a break event suchas the decoding of a software breakpoint instruction or the occurrenceof an analysis breakpoint/watchpoint. This state can also be enter by arequest from the host processor. A user can place the device into one ofthe other two states by giving the appropriate command to the debugger.

In this state, only time-critical interrupts can be serviced. No othercode can be executed. Maskable interrupts are considered time-criticalif they are enabled in the debug interrupt enable register (DBGIER). Ifthey are also enabled in the interrupt enable register (IER), they areserviced. The interrupt global mask bit (INTM) is ignored. NMI and RSare also considered time-critical, and are always serviced oncerequested. It is possible for multiple-interrupts to occur and beserviced while the device is in the debug-halt state.

Suspending execution adds only one cycle to interrupt latency. When theprocessor returns from a time-critical ISR, it reenters the debug-haltstate.

If a CPU reset occurs (initiated by RS), the device runs thecorresponding interrupt service routine until that routine clears thedebug enable mask bit (DBGM) in status register ST1. When a resetoccurs, DBGM is set, disabling debug events. To re-enable debug events,the interrupt service routine must clear DBGM. Only then will theoutstanding emulation-suspend condition be recognized.

Note: Should a time-critical interrupt occur in real-time mode at theprecise moment that the debugger receives a RUN command, thetime-critical interrupt will be taken and serviced in its entiretybefore the CPU changes states.

Single-instruction state 303. This state is entered when a user tellsthe debugger to execute a single instruction by using a RUN 1 command ora STEP 1 command. The CPU executes the single instruction pointed to bythe PC and then returns to the debug-halt state (it executes from oneinterrupt boundary to the next).

If an interrupt occurs in this state, the command used to enter thisstate determines whether that interrupt can be serviced. If a RUN 1command was used, the CPU can service the interrupt. If a STEP 1 commandwas used, the CPU cannot, even if the interrupt is NMI or RS. Inreal-time mode, if the DBGM bit is 1 (debug events are disabled), a RUN1 or STEP 1 command forces continuous execution of instructions untilDBGM is cleared.

Note: If a user single-steps an instruction in real-time emulation modeand that instruction sets DBGM, the CPU continues to executeinstructions until DBGM is cleared. If a user wants to single-stepthrough a non-time-critical interrupt service routine (ISR), a user mustinitiate a CLRC DBGM instruction at the beginning of the ISR. Once DBGMis cleared, a user can single-step or place breakpoints.

Run state 304. This state is entered when a user uses a run command fromthe debugger interface. The CPU executes instructions until a debuggercommand or a debug event returns the CPU to the debug-halt state.

The CPU can service all interrupts in this state. When an interruptoccurs simultaneously with a debug event, the debug event has priority;however, if interrupt processing began before the debug event occurred,the debug event cannot be processed until the interrupt service routinebegins.

Notice that the processor cannot pass directly between thesingle-instruction and run states. Notice also that the CPU can beobserved in the debug-halt state and in the run state. In thesingle-instruction state, the contents of CPU registers and memory arenot updated in the debugger display. In the debug-halt and run states,register and memory values are updated unless DBGM=1. Maskableinterrupts occurring in any state are latched in the interrupt flagregister (IFR).

Caution about breakpoints within time-critical interrupt serviceroutines:

Do not use breakpoints within time-critical interrupt service routines.They will cause the device to enter the debug-halt state, just as if thebreakpoint were located in normal code. Once in the debug-halt state,the CPU services requests for RS, NMI, and those interrupts enabled inthe DBGIER and the IER.

After approving a maskable interrupt, the CPU disables the interrupt inthe IER. This prevents subsequent occurrences of the interrupt frombeing serviced until the IER is restored by a return from interrupt(IRET) instruction or until the interrupt is deliberately re-enabled inthe interrupt service routine (ISR). Do not re-enable that interrupt'sIER bit while using breakpoints within the ISR. If a user does so andthe interrupt is triggered again, the CPU performs a new context saveand restarts the interrupt service routine.

FIG. 6 is a graphical summary of the differences between the executionstates of stop mode and real-time mode. Table 1 is a summary of howinterrupts are handled in each of the states of stop mode and real-timemode.

TABLE 1 Interrupt Handling Information By Mode and State If This ModeState Interrupt Occurs ... The Interrupt Is ... Stop Debug-halt RS Notserviced NMI Not serviced Maskable interrupt Latched in IFR but notserviced Single- RS If running: Serviced instruction If stepping: Notserviced NMI If running: Serviced If stepping: Not serviced Maskableinterrupt If running: Serviced If stepping: Latched in IFR but notserviced Run RS Serviced NMI Serviced Maskable interrupt Serviced Real-Debug-halt RS Serviced time NMI Serviced Maskable interrupt Iftime-critical: Serviced. If not time-critical: Latched in IFR But notserviced Single- RS If running: Serviced instruction If stepping: Notserviced NMI If running: Serviced If stepping: Not serviced Maskableinterrupt If running: Serviced If stepping: Latched in IFR but notserviced Run RS Serviced NMI Serviced Maskable interrupt Serviced

Note: Unless a real-time operating system is being used, the real-timeoperating system interrupt (RTOSINT) should not be enabled. RTOSINT iscompletely disabled when bit 15 in the IER is 0 and bit 15 in the DBGIERis 0.

Generally, a program uses the IRET instruction to return from aninterrupt. The IRET instruction restores all the values that were savedto the stack during the automatic context save. In restoring statusregister-ST1 and the debug status register (DBGSTAT), IRET restores thedebug context that was present before the interrupt.

In some target applications, an application might have interrupts thatmust not be returned from by the IRET instruction. Not using IRET cancause a problem for the emulation logic, because the emulation logicassumes the original debug context will be restored. The abort interrupt(ABORTI) instruction is provided as a means to indicate that the debugcontext will not be restored and the debug logic needs to be reset toits default state. As part of its operation, the ABORTI instruction:

Sets the DBGM bit in, ST1. This disables debug events.

Modifies select bits in, DBGSTAT. The effect is a resetting of the debugcontext. If the CPU was in the debug-halt state before the interruptoccurred, the CPU does not halt when the interrupt is aborted. The CPUautomatically switches to the run state. If a user want to abort aninterrupt, but keep the CPU halted, insert a breakpoint after the ABORTIinstruction.

The ABORTI instruction does not modify the DBGIER, the IER, the INTMbit, or any analysis registers (for example, registers used forbreakpoints, watchpoints, and data logging).

Real-time and Stop Mode Execution State Machine Transitions

A low logic level applied to the nTRST terminal or a software directiverequesting functional run forces the execution state to EXE 300. Thisstate directs the pipeline to fetch and execute instructions and processinterrupts in a normal way. Real-time debug allows the suspension ofprogram execution at points defined by breakpoints, watchpoints, anddebug software directives, provided the application defines these pointswithin an allowable stopping window in a software program to bedebugged.

Processing a debug event occurs during an allowable stopping window,i.e., when all the following conditions are met:

A debug event is present;

The instruction decode is at an instruction boundary;

Execution State Machine state is EXE;

Reset and non-maskable interrupt (NMI) are not active;

The execute command is not functional run;

DBGM bit enables debug exceptions; and

INT disables interrupts or a high priority (HP) interrupt request is notactive.

Debug request processing causes program execution to suspend at aninstruction boundary and the execution state machine state changes fromEXE state 300 to DSUSP state 310. When any of the debug above conditionsare not met, the execution state remains EXE and no debug eventprocessing occurs. The debug architecture permits debug event processingin the delayed slots of delayed instructions.

Debug events occurring outside the stopping window create a debugpending condition. This condition suspends program execution when theapplication enables debug interrupts by opening the stopping window.

DSUSP state 310 indicates instruction execution is inactive. Both theIDS and EXE states can generate entry into the DSUSP state and the DSUSPstate generates entry into both the IDS and EXE states. Interruptprocessing causes exit to the IDS state while debug software initiatedexecute directives cause exit to the EXE state.

High Priority (HP) interrupt processing refers to real-time, timecritical interrupts. DSUSP state 310 enables HP interrupts independentof the value of INT or software run directives. An interrupt pipelinejam for an HP interrupt occurs in response to an interrupt by selectingan interrupt decode input on mux 411, see FIG. 7. Concurrently, theexecution state is moved to IDS 320. This jam also causes an extra wordto be pushed on the stack. This word contains debug status describingthe reason the DSUSP state entry occurred. This word is the repeatcounter with additional register bits associated for holding additionalprogram counter and the debug status. When a debug event causing theDSUSP state occurs within a delayed slots of delayed branch and callinstruction, this value contains the program counter value generated bythe delayed branch or call along with debug status information. When adebug event occurs outside a delay slot, the value contains the repeatcounter value and the debug status information.

The presence of a run directive creates a slightly more complicatedscenario. A run directive enables low priority (LP) interrupt requestsand initiates fetching instructions. A run directive allows interruptswhile the pipeline refills as the program counter does not advance whilethe pipeline refill progresses. This minimizes the interrupt latency atall times and allows back to back interrupts. An HP interrupt request orNMI can occur at any time after the run directive. An LP interruptrequest can be processed after the run directive provided the INT enablebit is true.

One of four scenarios unfold after the run directive:

1) A reset is processed;

2) An HP (foreground) interrupt is processed;

3) An LP (background) interrupt is processed as a result of the run orstep directive, or;

4) The instruction referenced by the initial program counter beginsexecution by passing into the decode stage of the pipeline that isirrevocable.

Processing an HP interrupt causes the execution state to change to IDS.The IDS state differs from the EXE state in that the interruptprocessing creates a thread, linking the execution to the DSUSP state.

Processing reset, an LP interrupt, or the irrevocable decode of theinstruction fetch while in the IDS state moves the execution state toEXE. Execution state changes occur on the same clock the pipeline reset,interrupt jam, or instruction decode occurs. As described before, a tagidentifies the interrupt request as HP or LP. This tag allows theinterrupt jam to set the appropriate execute state. This generallyoccurs before the generation of the interrupt vector read address.

Debug Suspend State

When processor core 15 is in debug suspend state 310, it is notexecuting background code (when debugging in stop mode, all code isconsidered background code). Actions such as decoding a softwarebreakpoint instruction, the occurrence of an analysisbreakpoint/watchpoint, or at the request of the host are referred to asdebug events. When a debug event causes the device to enter the debugstate, it is also referred to as a break event. Analysisbreakpoint/watchpoints and test host requests are referred to ashardware break events, whereas the software break instruction is asoftware break event. In real-time mode, hardware break events onlyoccur when the debug mode (DBGM) in ST1 is enabled.

The debug mode (DBGM) bit of ST1 is very similar to INTM. It is activelow (i.e. a value of 0 indicates that the debug mode is enabled, a valueof 1 indicates it is disabled). DBGM is set to 1 on an interrupt. Itwill be restored on returning from the interrupt. It is initialized to 1at reset. Unlike INTM, DBGM is also initialized to 1 by ABORTI. It iscontrolled through the SETC/CLRC instructions. It can be set or clearedin parallel with the INTM bit (this is commonly done).

In the debug state, the program counter (PC) and status bits are held totheir values prior to the debug-event. The PC points to the instructionat which the debug-event occurred; that instruction is not consideredexecuted (this is the type of behavior exhibited when taking aninterrupt). When execution resumes, that instruction and those followingit need to be re-fetched.

If the device is halted in real-time mode and a reset occurs (but not aJTAG Test-Logic-Reset), the device runs from a reset vector until DBGMis enabled, which causes the outstanding emulation suspend condition tobe recognized. This is because the emulation logic which tracks this isonly initialized by the JTAG Test-Logic-Reset. A debug informationvisible through ST1 is cleared by the CPU's reset.

Software breakpoint instructions, ESTOP0 and ESTOP1, will cause thedevice to halt regardless of the setting of the DBGM bit, although it isnot recommended that these instructions be used in this manner.

FIG. 7 is a block diagram of core processor 15 of FIG. 1. Program memory400 holds instructions which form a software program. A first decodestage 410 in an instruction pipeline receives an instruction fromprogram memory 400 in response to program address bus PADRS. A seconddecode stage 420 further decodes an instruction after it leaves decodestage 410. Execution Control circuitry 430 is connected to test host 13to receive debug commands and is connected to decode stage 410 tocontrol execution of core processor 15.

Interrupting the Debug State

Interrupts are hardware- or software-driven signals that cause theprocessor to suspend its current program sequence and execute asubroutine. Typically, interrupts are generated by hardware devices thatneed to give data to or take data from the processor (for example, AIDand D/A converters and other processors). Interrupts can also signalthat a particular event has taken place (for example, a timer hasfinished counting).

On the processor, interrupts can be triggered by software (the INTR, ORIFR, or TRAP instruction) or by hardware (a pin, a peripheral, oron-chip logic). If hardware interrupts are triggered at the same time,the processor services them according to a set priority ranking. Each ofthe processor interrupts, whether hardware or software, can be placed inone of the following two categories:

Maskable interrupts. These are interrupts that can be blocked (masked)or enabled (unmasked) through software.

Non-maskable interrupts. These interrupts cannot be blocked. Theprocessor will immediately approve this type of interrupt and branch tothe corresponding subroutine. All software-initiated interrupts are inthis category.

The processor handles interrupts in four main phases:

1) Receive the interrupt request. Suspension of the current programsequence must be requested by a software interrupt (from program code)or a hardware interrupt (from a pin or an on-chip device).

2) Approve the interrupt. The processor must approve the interruptrequest. If the interrupt is maskable, certain conditions must be met inorder for the processor to approve it. For non-maskable hardwareinterrupts and for software interrupts, approval is immediate.

3) Prepare for the interrupt service routine and save register values.The main tasks performed in this phase are:

Complete execution of the current instruction and flush from thepipeline any instructions that have not reached the decode 2 phase.

Automatically save most of the current program context by s0aving thefollowing registers to the stack: ST0, T, AL, AH, PL, PH, AR0, AR1, DP,ST1, DBGSTAT, and IER.

Fetch the interrupt vector and load it into the program counter (PC).

4) Execute the interrupt service routine. The processor branches to itscorresponding subroutine called an interrupt service routine (ISR). Theprocessor branches to the address (vector) that was stored at apredetermined vector location and executes the corresponding ISR.

The processor of FIG. 7 supports 32 interrupt vectors, including thereset vector. Each vector is a 22-bit address that is the start addressfor the corresponding interrupt service routine (ISR). Each vector isstored in 32 bits at two consecutive addresses. The location at thelower address holds the 16 least significant bits (LSBs) of the vector.The location at the higher address holds the 6 most significant bits(MSBs) right-justified. When an interrupt is approved, the 22-bit vectoris fetched, and the 10 MSBs at the higher address are ignored.

Table 2 lists the available interrupt vectors and their locations. Theaddresses are shown in hexadecimal form. The table also shows thepriority of each of the hardware interrupts.

TABLE 2 Interrupt Vectors and Priorities Hard- Absolute Address ware(hexadecimal) Prior- Vector VMAP = 0 VMAP = 1 ity Description RESET 000000 3F FFC0  1 Reset (high- est) INT1 00 0002 3F FFC2  5 Maskableinterrupt 1 INT2 00 0004 3F FFC4  6 Maskable interrupt 2 INT3 00 0006 3FFFC6  7 Maskable interrupt 3 INT4 00 0008 3F FFC8  8 Maskable interrupt4 INT5 00 000A 3F FFCA  9 Maskable interrupt 5 INT6 00 000C 3F FFCC 10Maskable interrupt 6 INT7 00 000E 3F FFCE 11 Maskable interrupt 7 INT800 0010 3F FFD0 12 Maskable interrupt 8 INT9 00 0012 3F FFD2 13 Maskableinterrupt 9 INT10 00 0014 3F FFD4 14 Maskable interrupt 10 INT11 00 00163F FFD6 15 Maskable interrupt 11 INT12 00 0018 3F FFD8 16 Maskableinterrupt 12 INT13 00 001A 3F FFDA 17 Maskable interrupt 13 INT14 00001C 3F FFDC 18 Maskable interrupt 14 DLOGINT* 00 001E 3F FFDE 19Maskable data (low- log interrupt est) RTOSINT* 00 0020 3F FFE0  4Maskable real-time operating system interrupt Reserved 00 0022 3F FFE2 2 Reserved NMI 00 0024 3F FFE4  3 Non-maskable interrupt ILLEGAL 000026 3F FFE6 — Illegal-instruction trap USER1 00 0028 3F FFE8 —User-defined software interrupt USER2 00 002A 3F FFEA — User-definedsoftware interrupt USER3 00 002C 3F FFEC — User-defined softwareinterrupt USER4 00 002E 3F FFEE — User-defined software interrupt USER500 0030 3F FFF0 — User-defined software interrupt USER6 00 0032 3F FFF2— User-defined software interrupt USER7 00 0034 3F FFF4 — User-definedsoftware interrupt USER8 00 0036 3F FFF6 — User-defined softwareinterrupt USER9 00 0038 3F FFF8 — User-defined software interrupt USER1000 003A 3F FFFA — User-defined software interrupt USER11 00 003C 3F FFFC— User-defined software interrupt USER12 00 003E 3F FFFE — User-definedsoftware interrupt *Interrupts DLOGINT and RTOSINT are generated by theemulation logic internal to the core.

INT1-INT14 are 14 general-purpose interrupts. DLOGINT (the data loginterrupt) and RTOSINT (the real-time operating system interrupt) areavailable for emulation purposes. These interrupts are supported bythree dedicated registers: the interrupt flag register (IFR), theinterrupt enable register (IER), and the debug interrupt enable register(DBGIER).

The 16-bit IFR contains flag bits that indicate which of thecorresponding interrupts are pending (waiting for approval from theCPU). The external input lines INT1-INT14 are sampled at every CPU clockcycle. If an interrupt signal is recognized, the corresponding bit inthe IFR is set and latched. For DLOGINT or RTOSINT, a signal sent by thecore's on-chip analysis logic causes the corresponding flag bit to beset and latched. A user can set one or more of the IFR bits at the sametime by using the OR IFR instruction.

The interrupt enable register (IER) and the debug interrupt enableregister (DBGIER) each contain bits for individually enabling ordisabling the maskable interrupts. To enable one of the interrupts inthe IER, set the corresponding bit in the IER; to enable the sameinterrupt in the DBGIER, set the corresponding bit in the DBGIER. TheDBGIER indicates which interrupts can be serviced when the core is inthe real-time emulation mode.

The maskable interrupts also share bit 0 in status register ST1. Thisbit, the interrupt global mask bit (INTM), is used to globally enable orglobally disable these interrupts. When INTM=0, these interrupts areglobally enabled. When INTM=1, these interrupts are globally disabled. Auser can set and clear INTM with the SETC INTM and CLRC INTMinstructions, respectively.

After a flag has been latched in the IFR, the corresponding interrupt isnot serviced until it is appropriately enabled by two of the following:the IER, the DBGIER, and the INTM bit. As shown in Table 3, therequirements for enabling the maskable interrupts depend on theinterrupt-handling process used. In the standard process, which occursin most circumstances, the DBGIER is ignored. When the processor is inreal-time emulation mode and the CPU is halted, a different process isused. In this special case, the DBGIER is used and the INTM bit isignored. (If the DSP is in real-time mode and the CPU is running, thestandard interrupt-handling process applies.)

Once an interrupt has been requested and properly enabled, the CPUprepares for and then executes the corresponding interrupt serviceroutine.

TABLE 3 Requirements for Enabling a Maskable InterruptInterrupt-Handling Process Interrupt Enabled If ... Standard INTM = 0and bit in IER is 1 DSP in real-time mode and Bit in IER is 1 and bit inCPU halted DBGIER is 1

As an example of varying interrupt-enable requirements, suppose a userwants interrupt INT5 enabled. This corresponds to bit 4 in the IER andbit 4 in the DBGIER. Usually, INT5 is enabled if INTM=0 and IER(4)=1. Inreal-time emulation mode with the CPU halted, INT5 is enabled ifIER(4)=1 and DBGIER(4)=1.

FIG. 8 shows the IFR. If a maskable interrupt is pending (waiting forapproval from the CPU), the corresponding IFR bit is 1; otherwise, theIFR bit is 0. To identify pending interrupts, use the PUSH IFRinstruction and then test the value on the stack. Use the OR IFRinstruction to set IFR bits, and use the AND IFR instruction to clearpending interrupts. When a hardware interrupt is serviced, or when anINTR instruction is executed, the corresponding IFR bit is cleared. Allpending interrupts are cleared by the AND IFR, #0 instruction or by ahardware reset.

Notes: When an interrupt is requested by the TRAP instructions, if thecorresponding IFR bit is set, the CPU does not clear it automatically.If an application requires that the IFR bit be cleared, the bit must becleared in the interrupt service routine.

Bits 15 and 14 of the IFR correspond to the interrupts RTOSINT andDLOGINT, as described in Table 4. IFR bits INT1-INT14, are alsodescribed in Table 4.

TABLE 4 Interrupt Flag Register, IFR, RTOSINT Real-time operating systeminterrupt flag Bit 15 RTOSING = 0 RTOSINT is not pending RTOSINT = 1RTOSINT is pending DLOGINT Data log interrupt flag Bit 14 DLOGINT = 0DLOGINT is not pending DLOGINT - 1 DLOGINT is pending INTx Interrupt xflag (x = 1, 2, 3, . . . , or 14) Bit (x − 1) INTx = 0 INTx is notpending. INTx = 1 INTx is pending.

FIG. 9 shows the IER. To enable an interrupt, set its corresponding bitto 1. To disable an interrupt, clear its corresponding bit to 0. Twosyntaxes of the MOV instruction allow a user to read from the IER andwrite to the IER. In addition, the OR IER instruction enables a user toset IER bits, and the AND IER instruction enables a user to clear IERbits. When a hardware interrupt is serviced, or when an INTR instructionis executed, the corresponding IER bit is cleared. At reset, all the IERbits are cleared to 0, disabling all the corresponding interrupts.

Note: When an interrupt is requested by the TRAP instruction, if thecorresponding IER bit is set, the CPU does not clear it automatically.If an application requires that the IER bit be cleared, the bit must becleared in the interrupt service routine.

Note: When using the AND IER and OR IER instructions, make sure thatthey do not modify the state of bit 15 (RTOSINT) unless a real-timeoperating system is present.

Bits 15 and 14 of the IER enable or disable the interrupts RTOSINT andDLOGINT, as described in Table 5. IER bits INT1-INT14, are alsodescribed in Table 5.

TABLE 5 Interrupt Enable Register, IER RTOSINT Real-time operatingsystem interrupt enable bit Bit 15 RTOSINT = 0 RTOSINT is disabled.RTOSINT = 1 RTOSINT is enabled. DLOGINT Data log interrupt enable bitBit 14 DLOGINT = 0 DLOGINT is disabled. DLOGINT = 1 DLOGINT is enabled.INTx Interrupt x enable bit (x = 1, 2, 3, . . . , or 14) Bit (x − 1)INTx = 0 INTx is disabled. INTx = 1 INTx is enabled.

FIG. 10 shows the DBGIER, which is used only when the CPU is halted inreal-time emulation mode. An interrupt enabled in the DBGIER is definedas a time-critical interrupt. When the CPU is halted in real-time mode,the only interrupts that are serviced are time-critical interrupts thatare also enabled in the IER. If the CPU is running in real-timeemulation mode, the standard interrupt-hahdling process is used and theDBGIER is ignored.

As with the IER, a user can read the DBGIER to identify enabled ordisabled interrupts and write to the DBGIER to enable or disableinterrupts. To enable an interrupt, set its corresponding bit to 1. Todisable an interrupt, set its corresponding bit to 0. Use the PUSHDBGIER instruction to read from the DBGIER and the POP DBGIERinstruction to write to the DBGIER. At reset, all the DBGIER bits areset to 0.

Bits 15 and 14 of the DBGIER enable or disable the interrupts RTOSINTand DLOGINT, as described in Table 6. DBGIER bits INT1-INT14, are alsodescribed in Table 6.

TABLE 6 Debug Interrupt Enable Register, DBGIER, RTOSINT Real-timeoperating system interrupt debug enable bit Bit 15 RTOSINT = 0 RTOSINTis disabled. RTOSINT = 1 RTOSINT is enabled. DLOGINT Data log interruptdebug enable bit Bit 14 DLOGINT = 0 DLOGINT is disabled. DLOGINT = 1DLOGINT is enabled. INTx Interrupt x debug enable bit (x = 1, 2, 3, . .. , or 14) Bit (x − 1) INTx = 0 INTx is disabled. INTx = 1 INTx isenabled.

The flow chart in FIG. 11 shows the standard process for handlinginterrupts. When more than one interrupt is requested at the same time,the processor services them one after another according to their setpriority ranking. See the priorities in Table 2.

FIG. 11 is not meant to be an exact representation of how an interruptis handled. It is a conceptual model of the important events. Whatfollowing list explains the steps shown in FIG. 11:

step 1101) Interrupt request sent to CPU. One of the following eventsoccurs:

One of the pins INT1-INT14 is driven low.

The core emulation logic sends to the CPU a signal for DLOGINT orRTOSINT.

One of the interrupts INT1-INT14, DLOGINT, and RTOSINT is initiated byway of the OR IFR instruction.

step 1102) Set corresponding IFR flag bit. When the CPU detects a validinterrupt in step 1, it sets and latches the corresponding flag in theinterrupt flag register (IFR). This flag stays latched even if theinterrupt is not approved by the CPU in step 1103.

step 1103) Interrupt enabled in IER? Interrupt enabled by INTM bit? TheCPU approves the interrupt only if the following conditions are true:

step 1103 a: The corresponding bit in the IER is 1.

step 1103 b: The INTM bit in ST1 is 0.

Once an interrupt has been enabled and then approved by the CPU, noother interrupts can be serviced until the CPU has begun executing theinterrupt service routine for the approved interrupt (step 1113).

step 1104) Clear corresponding IFR bit. Immediately after the interruptis approved, its IFR bit is cleared. If the interrupt signal is keptlow, the IFR register bit will be set again. However, the interrupt isnot immediately serviced again. The CPU blocks new hardware interruptsuntil the interrupt service routine (ISR) begins. In addition, the IERbit is cleared (in step 1110) before the ISR begins; therefore, aninterrupt from the same source cannot disturb the ISR until the IER bitis set again by the ISR.

step 1105) Empty the pipeline. The CPU completes any instructions thathave reached or passed their decode 2 phase in the instruction pipeline.Any instructions that have not reached this phase are flushed from thepipeline.

step 1106) Increment and temporarily store PC. The PC is incremented by1 or 2, depending on the size of the current instruction. The result isthe return address, which is temporarily saved in an internal holdregister. During the automatic context save (step 1109), the returnaddress is pushed onto the stack.

step 1107) Fetch interrupt vector. The PC is filled with the address ofthe appropriate interrupt vector, and the vector is fetched from thatlocation.

step 1108) Increment SP by 1. The stack pointer (SP) is incremented by 1in preparation for the automatic context save (step 1109). During theautomatic context save, the CPU performs 32-bit accesses, and the coreexpects 32-bit accesses to be aligned to even addresses by the memorywrapper. Incrementing SP by 1 ensures that the first 32-bit access doesnot overwrite the previous stack value.

step 1109) Perform automatic context save. A number of register valuesare saved automatically to the stack. These registers are saved inpairs; each pair is saved in a single 32-bit operation. At the end ofeach 32-bit save operation, the SP is incremented by 2. Table 7 showsthe register pairs and the order in which they are saved. The coreexpects all 32-bit saves to be even-word aligned by the memory wrapper.As shown in the table, the SP is not affected by this alignment.

TABLE 7 Register Pairs Saved and SP Positions for Context Saves Bit 0 ofStorage Address Save Register SP Starts at SP Starts at Operation¹ PairsOdd Address Even Address 1 ← SP position 1 before step 8 1st ST0 0 0 ←SP position before step 8 T 1 1 2nd AL 0 0 AH 1 1 3rd PL² 0 0 PH 1 1 4thAR0 0 0 AR1 1 1 5th ST1 0 0 DP 1 1 6th IER 0 0 DBGSTAT³ 1 1 7th Returnaddress 0 0 (low half) Return address 1 1 (high half) 0 ← SP position 0after save 1 1 ← SP position after save ¹All registers are saved aspairs, as shown. ²The P register is saved with 0 shift (CPU ignorescurrent state of the product shift mode bits, PM, in status register 0).³The DBGSTAT register contains special emulation information.

step 1110) Clear corresponding IER bit. After the IER register is savedon the stack in step 1109, the CPU clears the IER bit that correspondsto the interrupt being handled. This prevents reentry into the sameinterrupt. If an application wants to nest occurrences of the interrupt,have the ISR set that IER bit again.

step 1111) Set INTM and DBGM. Clear LOOP, EALLOW, and IDLESTAT. Allthese bits are in status register ST1. By setting INTM to 1, the CPUprevents maskable interrupts from disturbing the ISR. If a user wishesto nest interrupts, have the ISR clear the INTM bit. By setting DBGM to1, the CPU prevents debug events from disturbing time-critical code inthe ISR. If the user does not want debug events blocked, have the ISRclear DBGM.

The CPU clears LOOP, EALLOW, and IDLESTAT so that the ISR operateswithin a new context.

step 1112) Load PC with fetched vector. The PC is loaded with theinterrupt vector that was fetched in step 1107. The vector forcesprogram control to the ISR.

step 1113) Execute interrupt service routine. Here is where the CPUexecutes the program code prepared to handle the interrupt

Although a number of register values are saved automatically in step1110, if the ISR uses other registers, the ISR may need to save thecontents of these registers at the beginning of the ISR. These valuesmust then be restored before the return from the ISR.

If a user wants the ISR to inform external hardware that the interruptis being serviced, the IACK instruction can be to send an interruptacknowledge signal. The IACK instruction accepts a 16-bit constant as anoperand and drives this 16-bit value on the 16 least significant linesof the data-write bus, DWDB(15:0).

step 1114) Program continues. If the interrupt is not approved by theCPU, the interrupt is ignored, and the program continues uninterrupted.If the interrupt is approved, its interrupt service routine is executedand the program continues where it left off (at the return address).

Non-maskable interrupts cannot be blocked by any of the enable bits (theINTM bit, the DBGM bit, and enable bits in the IFR, IER, or DBGIER). Theprocessor immediately approves this type of interrupt and branches tothe corresponding interrupt service routine. There is one exception tothis rule: When the CPU is halted in stop mode (an emulation mode), nointerrupts are serviced.

The processor non-maskable interrupts include:

Software interrupts (the INTR and TRAP instructions).

Hardware interrupt NMI

Illegal-instruction trap

Hardware reset interrupt (RS)

The software interrupt instructions and NMI are described in thissection. The illegal-instruction trap and reset are described later.

The INTR instruction can be used to initiate one of the followinginterrupts by name: INT1-INT14, DLOGINT, RTOSINT and NMI. For example,the interrupt service routine for INT1 can be executed by using thefollowing instruction:

INTR INT1

Once an interrupt is initiated by the INTR instruction, how it ishandled depends on which interrupt is specified:

INT1-INT14, DLOGINT, and RTOSINT

These maskable interrupts have corresponding flag bits in the IFR. Whena request for one of these interrupts is received at an external pin,the corresponding IFR bit is set and the interrupt must be enabled to beserviced. In contrast, when one of these interrupts is initiated by theINTR instruction, the IFR flag is not set, and the interrupt is servicedregardless of the value of any enable bits. However, in other respects,the INTR instruction and the hardware request are the same. For example,both clear the IFR bit that corresponds to the requested interrupt.

NMI

Because this interrupt is non-maskable, a hardware request at a pin anda software request with the INTR instruction lead to the same events.These events are identical to those that take place during a TRAPinstruction.

FIG. 12 is a flow chart of interrupt processing while in the executestate. Step 510 checks that any interrupt, such as interrupt N, is setin the interrupt flag register, (IFR). If so, then step 520 determinesif that interrupt is enabled in the interrupt enable register (IER). Ifso, then step 530 determines if interrupts are globally enabled bychecking the INTM bit. If so, step 532 is taken which begins processingan interrupt service routine for interrupt N. Prior to executinginstructions, step 540 saves registers on the stack and step 542 setsthe HPI bit if a corresponding bit in the debug interrupt enableregister (DBGIER) indicates that interrupt N is a high priorityinterrupt.

In stop mode debug, all interrupts including NMI and RESET are blocked(although latched) while the device is halted in the debug state. Notethat in stop mode, when multiple instances of the same interrupts occurwithout the first instance being serviced, the later interrupts will belost.

FIG. 13 is a flow chart of interrupt processing while in the debugsuspend state. In real-time mode, should the processor core be in thedebug state and be interrupted by a high-priority interrupt, asindicated by a corresponding bit in the debug interrupt enable register(DBGIER) in step 630, then the interrupt during debug state bit (IDS) ofDebug Status is set (and HPI will be set) as soon as the interruptservice routine is started in step 640. The value of Debug Status savedon the stack by the context store will not have the IDS or HPIset—rather, those status bits will be set immediately after this. TheIDS bit is used to indicate when, upon returning from an interrupt,processor core 15 should re-enter the debug state. Upon returning fromthe interrupt, the PC and status will return to their state before theinterrupt (unless the interrupt service routine purposely modified theirvalues on the stack).

FIG. 14 is a flow chart of steps performed within a high priorityinterrupt service routine when a second high priority interrupt occurs.It is possible for multiple interrupts to occur and be serviced whilethe device is in debug state if interrupts are re-enabled, as in step715. Step 730 determines if the pending interrupt is in fact a highpriority interrupt by checking the corresponding bit in DBGIER. In thiscase, the IDS bit is not set in step 740.

HP interrupt service routines (ISR) that execute during the IDS statemust use the return from interrupt (RTI) or abort interrupt (ABORTI)instruction provided by the CPU architecture. These ISRs can return tothe DSUSP state or be interrupted. The debug thread maintains a pathback to the DSUSP state when the application concludes the ISR executionwith an RTI instruction. Reset, a functional run (disable debug)directive, or execution of an abort interrupt instruction (ABORTI)breaks this link, clears the HPI and IDS status bits and set theappropriate status in the execution status register.

An illegal-instruction trap cannot be blocked, not even duringemulation. Once initiated, an illegal-instruction trap operates the sameas a TRAP #19 instruction. As part of its operation, theillegal-instruction trap saves the return address on the stack. Thus,the offending address can be detected by examining this saved value.

When asserted, the reset input signal (RS) places the core into a knownstate. As part of a hardware reset, all current operations are aborted,the pipeline is flushed, and the CPU registers are reset as shown in.Then the RESET interrupt vector is fetched and the correspondinginterrupt service routine is executed. Although RS cannot be masked,there are some debug execution states in which RS is not serviced, asdescribed earlier.

Debug Status

Allowing for break events within high priority interrupts requires thatinformation about what caused each break event be remembered, since theinternal status information will change with each break event. The DebugStatus register, DBGSTAT, indicates whether and why the debug state wasentered. This register is saved/restored as part of the interruptcontext save/restore, even when an interrupt is taken while the deviceis not in the debug state. It is also accessible through the data memorylocation 0x0800. The Debug Status Register is described in Table 8. Bitswhich are not described as Read Only can be written to via a contextrestore.

TABLE 8 Debug Status Register: DBGSTST 15 IDLE_FLAG Read Only: Device isin the IDLE state. 14 BRK Set to indicate a STOP due to a hardwarebreakpoint. 13 BRK_PEND Set to indicate that a hardware breakpoint hasrequested a STOP, but that the STOP has not yet occurred. 12 ANASTOP Setto indicate a STOP due to an Analysis event 11 ESTOP1 Set to indicate aSTOP due to an ESTOP1 instruction. 10 ESTOP0 Set to indicate a STOP dueto an ESTOP0 instruction. 09 HPI Set to indicate a high priorityinterrupt (one enabled in DBGIER) is being serviced. Set by taking atime-critical hardware interrupt (one who's bit is set in DBGIER)regardless of whether the device was in the debug state when theinterrupt began. When this occurs, the previous value of HPI is pushedon the stack as part of DBGSTAT. This bit is modified by pops of DBGSTATfrom the stack (this is how this bit will be cleared upon returning froma time-critical interrupt). Cleared by reset or ABORTI. 08 IDS Indicatesthat the device was awakened from the debug state (DSUSP) by the currenttime-critical interrupt. Set by taking an interrupt while in the debugstate (only an interrupt who's bit is set in DBGIER can do cause this).Cleared by taking any interrupt while not in the debug state (whetherthat interrupt's bit is set in the DBGIER doesn't matter). This bit ismodified by pops of DBGSTAT from the stack (this is how this bit will becleared upon returning from the time-critical interrupt that awoke thedevice from the debug state). Cleared by reset or ABORTI. 07:06 EXSMRead Only: Current EXSM state: IDBG = 3 [1:0] IDS = 2 DSUSP = 1 EXE = 0.05 FXWORK Read Only: Set if the CPU's write to FXREG succeeded, clearedif write was blocked. 04:00 DFC[4:0] Read Only: Current DFC value. Setto −1 by SYNC_LOSS.

Debug Instructions

Several instructions are provided for use by test host 13 duringemulation, as described in Table 9. The operation of ESTOP0 and ESTOP1within target device 10 depends on whether an emulator, such as testhost 13, is connected. Target device 10 knows that test host 13 isconnected when one of three states, EXE_COND, EXE_STEP, and EXE_HALT, isapplied to the emulation control state machine. Conversely, whenEXE_CONT is applied, test host 13 is not connected. EXE_CONT can begenerated from TRST- and via debug software control from test host 13.

TABLE 9 Debug Instructions ESTOP0 Used for a software breakpoint. Whenan emulator is connected, the ESTOP0 instruction stops processorexecution with the PC pointing to the ESTOP0 address. Otherwise, anESTOP0 advances the PC past the ESTOP0 instruction. ESTOP1 Used for aembedded breakpoint. When an emulator is connected, the ESTOP1instruction stops processor execution with the PC pointing to theinstruction after the ESTOP1. Otherwise, an ESTOP1 advances the PC pastthe ESTOP1 instruction. EALLOW Enables access to memory-mapped emulationregisters. The emulator can override EALLOW by setting a scan-only bitwhich will block writes regardless of EALLOW. The EALLOW bit is in ST1.This is a four cycle instruction. EDIS Disables access to memory-mappedemulation registers. This is a four cycle instruction. ABORTI Indicatesthat an ISR will not be returned from. This clears the IDS and HPI bitsin the DBGSTAT register, sets DBGM, and resets the Debug Frame Counter.This will cause the debug software to disregard any and all outstandingdebug states. Executing an ABORTI essentially resets the debuginformation. PUSH Writes the current value of the DBGIER instructionDBGIER onto the stack. POP Reads a value from the stack and places it inDBGIER the DBGIER register.

Aborting from Interrupts

In some target device applications, there will be interrupts which arenot returned from in the normal manner. This can cause a problem fordebug, since register bits such as HPI, IDS, and DFC make assumptionsthat returning from an interrupt will “put things back” the way theywere. The ABORTI instruction is provided as a means to indicate that aninterrupt is not going to be returned from, and causes the emulationcircuitry on core processor 15 to be reset to it's default state. TheABORTI instruction clears IDS and HPI in the DBGSTAT register. Thismeans if core processor 15 was in the debug state when the interruptoccurred, it will not return to the debug state. The ABORT instructiondisables DBGM in status register ST1. The debug frame counter isre-initialized. This means that if there were multiple debug statesoutstanding (something that will only occur when break events occurwithin time-critical interrupts) they will be forgotten—debugging isstarting anew.

The ABORTI instruction does not modify the DBGIER register; it does notaffect non-debug registers such as INTM or IER; nor does it clear orchange the analysis registers relating to Breakpoints, Watchpoints,Benchmark Counter, and PSAs or High Speed Data Logging.

Debug Architecture Embodiment

An embodiment of the present invention is referred to as ICEMaker.™ Thisarchitecture creates a set of capability that enables sophisticatedsystem debug and test. This debug architecture, composed of severalbuilding blocks, provides a scaleable set of capability. The buildingblocks have both mandatory and optional components. The mandatorycomponents cannot be excluded from a building block. A building blocksdescriptions identify capability as mandatory or optional.

Building blocks are architecture independent. Most if not all buildingblocks require a wrapper to connect them to other CPU components. Forinstance, many of the registers used for debug are memory mapped. Theregister is in the generic component while the placement of the registerin the memory map and the logic needed to read the register is in thewrapper. Models for the generic component can be easily ported fromdesign to design, modulated to optimize bus widths when required.

FIG. 15 is a block diagram of an embodiment of emulation circuitryaccording to the present invention, the ICEMaker Unit (IMU) architectureshowing both the fixed and variable portions of the components. Thewrapper portion 800 of the ICEMaker function is device specific,completing the creation of specified ICEMaker functions by adapting thestandard architecturally independent portion of the function to aspecific CPU architecture. Debug core 810 contains all componentsnecessary for interacting with a test host. Other ICEMaker buildingblocks are listed in Table 10.

TABLE 10 ICEMaker Building Blocks Debug Core 810 The capability requiredto create a debug tool according to aspects of the present inventionAddress Comparison Provides event, break, data logging, Unit 820 PSA,and counter functions Data Comparison Provides event, break and PSA Unit830 and counter functions External Comparison Provides break functionsfor external Unit 840 and trigger channel inputs Control Specificcomponents such as trigger Elements 860 channel or execution control

FIG. 16 is a block diagram illustrating the functional partitioning ofthe IMU. Debug unit 900 provides a connection to core CPU 910 buses andpipeline. This unit also contains the debug registers such as ACU 820,DCU 830, and ECU 840 used to access CPU registers and memory. CEs 920are a collection of control functions for DU logic and registers andother blocks such as CPU execution control. Interface adaptor (IA) 930provides a bridge between the IEEE scan and test access port activity(TAP) at ICEMaker port and the CE/DU logic.

The IA is a generic function and is CPU core independent. Thiscapability includes a IEEE 1149.1 Test Access Port (TAP), very basicscan capability, and the TDO output management. Interface adapter 930supplies glitch free decoded tap signals to the remainder of the systemincluding ASIC logic external to the CPU MegaModule. System logicexternal to the CPU MegaModule can use IEEE op-codes allocated to systemfunctions to create customized IEEE functions such a boundary scan orsimilar capability. The 38 bit instruction register value providesnumerous unused op-code groups that can be used in other parts of adesign. This allows ASIC modules to share the use of the MegaModule IEEEscan and TAP facilities.

A 38-bit scan path consisting of a 6 bit extended shift register (EXSR)and 32 bit general purpose shift register (GPSR) creates the IA'sinstruction scan path. The GPSR supplies a 32 bit data value while theEXSR identifies the instruction class (op-code). Data scans use the 32bit GPSR in several configurations. The EXSR forms a six bit counter fordata scans, creating markers for specialized transfers. This counter,coupled with special GPSR scan path configuration, provides extremelyhigh bandwidth production test upload and download sequences, receivingdata four bits at a time from the TDI_i, TDO_i, nET1_i, and nET0_iterminals or transmitting data four bits at a time from the TDI_o,TDO_o, nET1_o, and nET0_o terminals for uploads.

The CEs 920 block contains a number of state machines and thecombinatorial logic necessary to manage debug functions. The CE logicincludes all debug control needed to manage the DU function. Thisincludes state machines and combinatorial logic needed to managecapabilities such as breakpoints, counters, benchmarking, and datalogging. This block defines IEEE op-code usage and scan paths subject toguidelines established in this document. CE logic suppliessynchronization between TCK and FCK logic. Since there are number ofvaried CE components, modeling them separately allows optimum physicalplacement within the MegaModule.

The CE functionality includes but is not limited to:

IEEE instruction register controls;

IEEE instruction register decode;

Synchronization between the TCK and FCK clock domains;

Status acquisition;

Scan framework;

CPU execution control;

DMA control;

Debug memory read and write control;

Event generation logic;

Miscellaneous DU control; and

Trigger channels.

Debug Unit (DU) 900 is the debug and test connection to the CPU 910 datapath. It is tightly coupled to the CPU and its buses. The DUfunctionality is portable to varied architectures but tailored to matchCPU address and data bus widths. Since it connects to most of the CPUdata path, it is logical that the CPU datapath includes the DU. Itsfunctionality includes: memory access; register access; multiplebreakpoints; benchmarking and event counting facilities; timers; datalogging; DMA address and data management; communication register; CPUCore Signature Analysis, user definable signature analysis, andexpedited test load/unload.

A combination of DU and CE logic provides access to CPU registers andmemory. The DU provides a dual interface, scannable and memory mapped,to debug facilities such as breakpoints, benchmarking, data logging, andPSAs.

FIG. 17 a more detailed block diagram of the emulation circuitry of FIG.15. The IMU contains three large blocks of logic as shown. These blocksare the Memory Unit (MU) 1000, Address Comparison Unit (ACU) 820 andData Comparison Unit (DCU) 830. The units are the backbone of the IMUand comprise the Debug Unit (DU). Control elements (CEs) 1010 supportthe DU and provide control functions for it. These elements include theACU 820, DCU 830 and ECU (External Control Unit) 840 control registers,not shown in FIG. 17. The MU provides the basic debug access to memoryand registers for execution control. Most if not all miscellaneousregister functions and control bits reside in this function. The ACUprovides Breakpoint, Counter, PSA and Data Logging support. The DCUprovides Breakpoint and PSA support. A third small unit, the ECUcontrols external inputs to the event functions.

ICEMaker Port 850 provides the interface to all CPU MegaModule scanpaths. All instruction and data scan paths of the MegaModule involve thememory unit. Two registers, a 32 bit General Purpose Shift Register(GPSR) and a six bit Extension Shift Register (EXSR) combine to providesupport for all instruction and data scan paths.

The GPSR scans during instruction and data scans. The EXSR scans duringinstruction scans and reconfigures as part of a six bit counter duringdata scans. This is practical as the EXSR is not part of any data scanstring. The CAPTURE_IR state loads both registers, the GPSR with thecontents of either the FXREG or system status and the EXSR with a ministatus. The mini status defines the contents of the GPSR one of severaldata forms.

The LD_REGS_0, and LD_REGS_1 instruction classes can load debug controlregisters within the IMU (MF_REGS_0 and MF_REGS_1) or just retrieve readdata or system status. The load of these registers occurs when the IDLEstate immediately follows an instruction scan defining these classes (aninstruction scan ending in IDLE). The 32 bit data field accompanying theop-code class designation provides the data used to load the MF_REGS_0and MR_REGS_1 registers. These registers partition into segments thatare individually loadable. Bits imbedded in the 32 bit data fieldcontrol the load of specific register segments in both the MF_REGS_1 andMF_REGS_0 registers. In other words, the 32 bit data value contains boththe register segment data and register segment load enables.

The MF_REGS_0 and MF_REGS_1 registers are in the FCK domain. Data isfirst loaded into the JXREG by the JTAG Update_IR state or itsequivalent. The JTAG state transition from the UPDATE_IR the IDLE state(accompanied by the MF_REGS_0 or MF_(—REGS)_1 op-code initiates thetransfer. Previously loaded JXREG data that has not been dispositioned(used) blocks the load of new JXREG data. Data moves from the JXREG tothe FXREG where it moves to the enabled register segments, or, in thecase of MF_REGS_1, generates strobes. Consequently, loading of theMF_REGS_1 and MF_REGS_0 segments inhibits changing the JXREG, FXREG andinstruction (IR_LTCH) registers until the transfer completes. Theseregisters can be loaded by the application when the special monitor modeis enabled at the MegaModule boundary. An assembly language registerwrite instruction first writes the data into the FXREG. From there it ismoved to the appropriate register. The application sees the MF_REGS_0and MF_REGS_1 registers in the memory map. A write to either registerfirst writes the data into the FXREG. From there it is moved to theappropriate register just as with scannable scenario. The write to theFXREG is transparent to the application software.

Table 11 shows the strobes that can be generated by writing to theMF_REG_0 register. A one in any bit position creates the specifiedaction while a zero creates no action. Table 12 contains a briefdescription of the MF_REGS_0 segments. Table 13 contains a briefdescription of the MF_REGS_1 segments. Table 14 describes the individualbits in the MCT register segment, while Table 15 describes the testsegment bits. The reset control register segment, shown in Table 16,provides reset modes for the CPU MegaModule. The real-time segment,shown in Table 17, provides reset modes for the CPU MegaModule.

TABLE 11 MF_REG_0 Generated Strobes Bit Mnemonic Type Description 31CMD_ERR FS Initialize command error 30 INIT_DFC FS Initialize DFC to allones. 29 IDLE_FLAG FS Clear Idle flag 28 NF_SUSP FS Clear new framesuspend 27 TF_SUSP FS Clear target frame suspend 26 SYNC_ERR FS Clearsynchronization loss 25:24 Reserved

TABLE 12 Miscellaneous Function Register Zero Segments Segment # BitsDescription Strobes 8 Strobes ETR 11  Emulation and Test Pin Control -Defines the ET pin operating modes. This field supports a five bitcontrol mechanism for each ETx pin plus a one bit segment select. RSM 7Run State Machine - This register segment is the run state machine. Thisfield supports a 5 bit RSM with two segment selects. The first qualifiesthe load of the segment and the second qualifies the load of the segmentbased on the CPU running or halted. DFR 6 Debug Frame Register - Definesthe debug frame. This field supports a one bit segment select and amaximum DFR of 6 bits

TABLE 13 Miscellaneous Function Register One Segments # of Segment BitsDescription MCT 8 MU Cycle Type Register - Defines the qualification(High priority and rudeness level for accesses generated by the memoryunit. This segment contains a seven bit register and a one bit segmentselect. This register segment is not implemented when the 7 bit datavalue generated by this register segment can be concatenated with theaddress and the total number of bits be less than or equal to 32 asshown in FIG. 5-5. Reserved 9 Reserved TST 8 Test Register- Defines thetest modes or clock source This segment contains a six bit register anda one bit segment select. RCR 5 Reset Control Register - Defines resetmodes and capabilities. This segment contains a four bit register and aone bit segment select RT 2 Real-time - This register segment containsthe real-time bit and a one bit segment select.

TABLE 14 MCT Segment Description Bit Name Bit # Description MCTEN 31 MCTEnable - MCT segment enable NOID 30 No ID - This bit, when a one, causesthe MU access to not be qualified by the DFC and DFR comparison. Whenthis bit is a zero, the DFC and DFR comparison qualifies the MU accessrequest. (No match is no request.) MUHP 29 MU High Priority - Memoryunit access high priority. When a one, the memory unit access request ishigh priority. When a zero the access request is low priority.. MHPIU 28MU DBG qualifier - When this bit is a one, the status register HPI bithas no influence on a MU access request. When a zero, the statusregister HPI bit must be a zero to generate a MU access request. MDBGU27 MU DBG qualifier - When this bit is a one, the status register DBGMbit has no influence on a MU access request. When a zero, the statusregister DBGM bit must be a one to generate a MU access request.CYC[2:0] 26:24 Cycle Type - Forms the three cycle code bits for MUcycles.

TABLE 15 TST Segment Description Bit Name Bit # Description TSTEN 23Test Segment Enable - This bit, when a one, allows the remainder of thetest segment to load from the FXREG. When this bit is a zero, the TESTsegment. CKSRC 22:21 Clock Source - This field selects a MegaModule[1:0] clock source for the functional logic. 00 - Functional clock 001 - Functional clock 1 10 - Test clock 11 - Test clock divided by twoTSTMAP 20 Test Map - Switch the memory map to the test configurationwhen this bit is a one. Use the normal memory map when a zero. ISOLATE19 Isolate - Disconnect the CPU MegaModule inputs when this bit is aone, supplying alternate inputs from the MegaModule. MegaModulegenerated stimuli connect to these inputs. Use the normal CPU MegaModuleinputs when this bit is a zero. Reserved 18:16 Reserved

TABLE 16 RCR Segment Description Bit Name Bit # Description RCREN 15Test Segment Enable - This bit, when a one, allows the remainder of thetest segment to load from the FXREG. When this bit is a zero, the RCRsegment does not load. RES_SET 14 Reset Set - Set the Res_Ltch when thisbit is a one. Do nothing when this bit is a zero. The reset disconnectfunction does not affect the function of this bit. The generation ofreset by this bit. RES_CLR 13 Reset Clear - Attempt to clear theRes_Ltch when this bit is a one. RES_DIS 12 Reset Disconnect -Disconnect the SE and system resets from the Res_Ltch when this bit is aone. Connect these resets when this bit is a zero. RES_STAY 11 ResetStay - Set the Res_Stay bit to the value of this bit. The reset staybit, when a one stretches an incoming reset indefinitely until this bitis set to a zero.

TABLE 17 RT Segment Description Bit Name Bit # Description RTEN 15Real-time Segment Enable - This bit, when a one, allows the remainder ofthe real-time segment to load from the FXREG. When this bit is a zero,the real-time segment does not load. RT 14 Real-time - This bit, when aone, places the CPU in real-time mode. When a zero it places the devicein stop mode.

The LD_DMA_WD instruction loads the JXREG with the data in the datafield of the instruction for use as write data by the internal DMA. Whenthe DMA is activated to do writes originating from the JTAG block, ituses this data to generate a memory writes. DMA notification of dataavailable occurs provided the instruction scan passes directly from theUpdate_IR state to the IDLE state and JXREG load of the write data wasnot blocked by a busy condition. When the DMA is not activated it hangsthe interface.

Debug software can use two mechanisms to get to system resources. Thisinstruction directly supports one of them, the MU option. The memoryunit (MU) provides register set (an address register and write dataregister) that debug software loads for each access. The addressregister doubles as the read data input register. Once the read isinitiated, the address is no longer needed so this does not present aproblem. The address register is loaded through the write data register.Debug software provides the address for each read access and initiatesthe read access with a single instruction scan. It must end in the IDLEstate, passing directly from the Update IR to the IDLE state to load theaddress.

A second read option, indirectly supported by this instruction, requiresthe address comparison unit (ACU) to supplement the MU registers withauto incrementing address generation. Debug software allocates the ACUto breakpoints, PSA generation, timing and benchmarking features, andDMA address generation. During data logging the application uses the ACUDMA facilities while production tests use the DMA facilities to expeditetest down loads. Because of the multifunction aspects of the ACU, it isnot generally used for low bandwidth transfers.

The SYS_ACC_W instruction directly supports the debug software's abilityto generate write accesses using the memory unit (MU) resources. Thememory unit (MU) provides register set (an address register and writedata register) that debug software loads for each access. The addressregister is loaded through the write data register, necessitating theaddress register be loaded first. The load of these two registers usesthe same instruction with different option fields. The instructionaddress size/type option is specified for the instruction scan thatloads the address. This instruction scan loads the address register(FXREG) but does not initiate an access. It must end in the IDLE stateto load the address. The second instruction scan uses one of the threedata size type options. These options specify the data size. Theunderlying hardware combines the two LSBs of address with the size/typeinformation to fully define the access. The second instruction scan mustalso end in IDLE. This instruction scan places the write data in thewrite register (JXREG).

The SYS_ACC_W instruction class provides directly for MU accesses. Itindirectly supports the block transfer capability as MU writes canactivate the ACU internal DMA. This gives debug access to block readsand writes using an auto-incrementing address mechanism in the ACU.Debug software defines the starting DMA address and activates the DMA(as opposed to the application doing it in data logging modes). Write(s)to the to the DMA address generator/word counter, activating the DMAready the DMA for read and write transfers using the JXREG, the normalwrite data buffer.

The MU and ACU memory access mechanisms share a write data buffer buthave separate read data buffers. This allows simultaneous reading fromboth mechanisms, read with one mechanism and write with the other.Writes can be interleaved by debug software but do not occursimultaneously (first one mechanism and then the other).

The test access instruction classes provides fast download (TEST_ACC_W)and upload (TEST_ACC_R) of memory in the production test environment.Both of these instruction classes reconfigure the scan chain for datascans. This supplies four scan inputs for the TEST_ACC_R instruction andfour scan outputs for the TEST_ACC_W instruction. TDI handles the mostsignificant byte, TDO_o the next, nET1, the next, and nET0 the leastsignificant byte for both reads and writes. The both reads and writesuse the ACU address mechanism to do block transfers. Instruction scansending in IDLE use the 32 bit data field of these two instructions toload the 16 LSBs of MF_REG_1 and the 16 bit ACNTL.

These instructions clear the AMSK, AREF, DCNTL, DMSK, DREF, and ECNTLregisters when the ACNTL register is loaded. This allows reset to beapplied, the clock source to be specified, and the initiation ofinternal DMA to occur simultaneously.

The JXREG sources the write data the writes, independent of the spacedefined by the ACNTL register. The FXREG receives all read data, fromprogram memory, data memory and auxiliary spaces. For reads, the ACUissues continuous read requests. This continues until the DMA actionterminates. A not ready condition occurs when the FXREG is loaded.Reading the FXREG data generates a ready for one clock and then theFXREG fills again. Since data scans export the read data, there is nomini status and a new word is exported every eight clocks. A nibble isoutput from the export pin set every TCK (every 5 ns at 200 MHz).Reaching the access limit specified by the word count or other haltmechanism ends the read requests. The cycles in the memory pipelinecontinue and the new read data moves to the FXREG as the FXREG emptiesinto the GPSR. Eventually the memory pipeline completely empties and nonew data is available. The EXSR counter loads the first word into theGPSR during the Capture_DR state and subsequently moves data from theFXREG to the GPSR every eight Shift_DR states after the initial capturestate. The MF_REGS_1 load must apply reset to the CPU, and specify TCK/2as the clock source.

Running through an example calculation, a TCK frequency of 200 MHz yielda functional clock frequency of 100 MHz. The 200 MHz TCK provides aninput or output bandwidth of 800 Mbits/sec. A 1000 32 bit word test casewould take approximately 40 usec to load. Assuming the CPU takes onaverage two clocks per word to execute this program, the CPU takesapproximately 20 us to complete the test execution. This makes the ratioof test load time to execution time approximately 2 to 1. This ratio isalso valid for unload times.

This instruction provides for the generation of TCK based strobes thatcoincide with the duration of the IDLE state following an instructionscan. It also provides high bandwidth transfers by allowing imbeddedcommands in a data scans.

This instruction provides for the initialization of miscellaneous errordetects, in addition to providing for the initialization of the commandinterface. Since other instructions can request actions that cannot becompleted, (i.e. the rudeness level of a memory access can be low enoughthat it is blocked), a mechanism to abort a request in progress isneeded. This allows the debug interface to change the access criteriabefore trying again.

Table 18 lists all the debug related registers for ICEMaker.Miscellaneous control bits supporting the JTAG interface are notincluded in this list. Most but not all of the debug unit registers areplaced in the memory map so they are accessible by both debug softwareand the application. There are three levels of register access:registers always shared by the application and debug facilities;registers accessed through the ICEMaker™ Port only; and registersaccessed through the ICEMaker™ Port or a specially privileged monitorprogram but not shared.

The application and debug software share registers controlling externaltrigger event inputs, breakpoints and watchpoints, data logging, PSA,and count functions. The application and debug software can notsimultaneously own these resources but establish ownership and releaseownership through memory mapped control registers continuously visibleto both the application and debug software. The debug software has theability to seize any resource if necessary, or negotiate with theapplication through software sequences.

Other registers are specific to JTAG scan support and can never beaccessed by the application. This class of registers is clocked with TCKand includes the JXREG, GPSR. EXSR, and IR_LTCH registers. Anotherregister, the MF_REGS_1 register is clocked with FCK but is notaccessible to the application. This register controls the deviceexecution mode (real-time and stop mode), special reset modes, testmodes, clock source selection and the like. The application could getsuicidal if granted access to this register.

A third class of registers is accessible through JTAG and accessible tothe application if a special privileges are granted to a monitorfunction via a CPU MegaModule terminal (MON_PRIV). When this terminal isgrounded the application cannot access this register class. When thisterminal is a logic one, the application code can access a debug controlregister normally controlled by JTAG scans. This register contains ETxpin control, execution control, the debug frame reference register.

TABLE 18 Debug Register List Memory Register Width Mapped NameDescription  8 No IR_LTCH Latched Instruction Register  6 No EXSRExtended Shift Register 32 No JXREG JTAG Transfer Register 32 No GPSRGeneral Purpose Shift Reg. 32  No** FXREG Functional Transfer Register32 No MF_REGS_1 Misc. Function Register 1 32 Yes MF_REGS_0 Misc.Function Register 0 16 Yes DBG_STATUS Debug status 16 Yes ECNTL ExternalEvent Control 16 Yes ACNTL Address Unit Control 32 Yes AMSK Adrs. MaskRegister 32 Yes AREF Adrs. Reference Register 16 Yes DCNTL Data UnitControl 32 Yes DMSK Data Mask Register 32 Yes DREF Data ReferenceRegister 16 Yes HPIR High Priority Interrupt Reg. **Monitor privilegedwrites to MF_REG_0 use the FXREG as a temporary register.

SUMMARY

Table 19 provides a summary of terms used herein. Table 20 provides asummary of difficult boundary conditions that relate to the executionthat state machine. Table 20 provides a summary of difficult conditionsthat relate to the IDLE instruction.

TABLE 19 Summary of Emulation and Architectural Terms and AbbreviationsBACK- Main body of code, generally not as time-critical as GROUND ISRroutines which service motor controls or high- CODE speed timers. BREAKA debug event which has been selected to halt the EVENT device.Typically, actions such as the decoding of an ESTOP, occurrence of ananalysis watchpoint, or request of the host, will halt the device. Ahardware break event is a break event other than an ESTOP. BREAK- SeeBREAKPOINT, HARDWARE. POINT, ANALYSIS BREAK- Analysis event whichtriggers when the instruction POINT, about to be performed is from acertain address. This HARD- is also referred to as an analysisbreakpoint. WARE BREAK- See ESTOP. POINT, SOFT- WARE BTT See XDS-524.DATA The rapid transfer of memory contents between the LOGGING deviceand a host, as initiated by the device application. DBGACK Core signalwhich indicates that the device is in debug mode. This has historicallybeen called SUSPEND, and it may be changed to this name to preventconfusion.. DBGM Debug Mode bit within ST1 which must be enabled toallow polite DT-DMA accesses and hardware break events. DFC, See DEBUGFRAME COUNTER and DEBUG DFR FRAME REGISTER. DT- Debug and Test DirectMemory Access. A DMA mechanism to access memory and registers withoutfetching CPU instructions from memory or requiring the CPU to stop.DEBUG Counter which keeps track of nested interrupts FRAME when thedevice is stopped in debug mode. COUNTER DEBUG Register which iscompared against the Debug FRAME Frame Counter to ensure that the devicestate being REGISTER accessed is what the debug software expects. DEBUGAn action such as the decoding of an ESTOP, EVENT occurrence of ananalysis watchpoint, or request of the host, which may result in specialdebug behavior, such as halting the device or pulsing one of theEMU0/EMU1 signals. Often times when “debug event” is mentioned insteadof a “break event”, it means the device will not enter the debug state.DEBUG State in which the device does not execute STATE application codebut may perform debug tasks. DEBUG Used to indicate how the debug ANDTEST resources are to be used. MODE DEVICE Device power-up mode,OPERATING typically used for test purposes. MODE DBGSTAT Debug Statusregister. This is saved/restored as part of the interrupt contextsave/restore. EALLOW Emulation instruction which enables access tomemory-mapped emulation registers. EDIS Emulation instruction whichdisables access to memory-mapped emulation registers. EMULATION Theduplication of a device's behavior while providing additional visibilityand control over it's normal operation. Methods of achieving thisinclude self-emulation and in-circuit emulation. EMULATION, Features ofa device which grant a user visibility and SELF- control over it's ownnormal operation. EPK Emulation Porting Kit. A software libraryproviding access and control to a device via the scan controller. Thehost debugger source code is available within the EPK. ESTOP Emulationinstruction whose decoding produces an event which causes the device toenter the debug state if an emulator is connected. Used to implementsoftware breakpoints. EMU0 Emulation and Test trigger channel 0. Usedfor asynchronous communications between the device and the scancontroller (and potentially other devices or equipment) EMU1 Emulationand Test trigger channel 1. See EMU0 above. EXE_COND Execution state inwhich instructions are performed until a break event occurs. EXE_CONTExecution state in which instructions are performed as if the emulatoris disconnect. EXE_HALT Execution state in which execution is suspend.Instructions are not fetched. EXE_STEP Execution state in which a singleinstruction is performed, and then execution is suspended. FLATTENINGPipeline flattening aligns the bus cycle activity for the programinstruction fetch, data operand reads, and data result write into asingle cycle. FLUSHING The act of discarding the contents of pipelinestages prior to the second decode while allowing the second decode andlater pipeline stages to complete. FORE- Time-critical code implementedas an interrupt- GROUND service routine. See Time-Critical Interrupt.CODE HPI High Priority Interrupt bit in ST1 indicates whether thecurrent interrupt is, a high priority interrupt. This can be used tooptionally block hardware break events. DBGIER Debug-enabled InterruptRegister. Indicates which interrupts are time critical. In realtimemode, time critical interrupts can interrupt the device while it is inthe debug state. HOLES Unused cycles which can be utilized for DT-DMA.Holes can occur naturally due to branches, or be inserted by highpriority DT-DMA requests. HOST A personal computer or workstation whichinteracts with the target system. In this document, “host” generallyrefers to the computer running the debugger software. HSTOP Emulation(external) event which causes the device to enter the debug state. ICETraditional abbreviation for “In-Circuit Emulator”. IDS Interrupt DuringDevice State bit within ST1 indicates whether device was interrupted bya high priority interrupt while it was in the debug state. If this bitis set when a RETI instruction is executed, the device will enter thedebug state upon returning. IDLE Low-power mode in which CPU operationis suspended. IDLE_FLAG Indicates a request for the device to enter thelow- power mode. IER Interrupt Enable Register. Part of the standard CPUarchitecture. IFR Interrupt Flag Register. Part of the standard CPUarchitecture. IMR Interrupt Mask Register - this is an older term - IERis now more commonly used. Part of the standard CPU architecture.IN-CIRCUIT A development system in which the device being EMULATORemulated is replaced by a development system which duplicates thefunctionality of the device while providing additional visibility andmemory configuration options, as well as control over the device'soperation. INTERRUPT Indicates a point at which an interrupt can betaken. BOUNDARY ISR Interrupt Service Routine. Part of the standard CPUarchitecture. JTAG Abbreviation for the IEEE Std. 1149.1, “Standard TestAccess Port and Boundary-Scan Architecture” which defines a protocolbased on a five-pin interface (TCK, TMS, TDI, TDO, and TRST-) used fortest and emulation. NMI Non-maskable interrupt. Part of the standard CPUarchitecture. NULL Micro-instruction which is similar to a NOP but doesnot increment the PC. PC Device program counter. Part of the standardCPU architecture. POLITE A DT-DMA access which does not occur if theDBGM DT-DMA is disabled. PSA Parallel Signature Analysis. Similar to acircular- redundancy check or checksum, a PSA is used to calculate aunique number given a sequence of values. By sampling bus activityduring a test case, it is possible to verify proper CPU operation atspeed. PRODUC- A device which is intended for volume production TION andsale. Cost (as determined by silicon size, DEVICE package type, testtime, and other factors) is of key importance. PULSE An action such asthe decoding of an ESTOP, EVENT occurrence of an analysis watchpoint, orrequest of the host, which causes a pulse on one of the EMU0/EMU1signals. REAL- Debug mode in which time-critical interrupts are TIMEstill serviced while application code and non-time- critical interruptsare not. RESET Device reset. Part of the standard CPU architecture. RUDEA DT-DMA access which ignores the DBGM setting. DT-DMA SCAN The hardwarewhich the host PC uses to control the CONTROL- JTAG interface of thedevice. The XDS-510 (ISA LER board for PCs), XDS-510WS (SCSI peripheralfor Suns and HPs), and XDS-510PP (PC parallel port peripheral) are scancontrollers available from TI. STEP Execution of a single applicationcode instruction before returning to the debug mode. STOP Debug mode inwhich no interrupts are serviced and MODE application code is notexecuted. SUSPEND See DBGACK. TARGET The device being emulated and/ortested. The term “target” comes a military phrase meaning “somethinglikely to be destroyed”. TCK JTAG Test Clock. Used to drive the IEEE1149.1 state machine. TDI JTAG Test Data Input. Provides scan datainput. TDO JTAG Test Data Output. Provides scan data output. TIME- Aninterrupt which must be serviced within a CRITICAL certain time limit,even while debugging the INTERRUPT application. To indicate that aninterrupt is time- critical, the appropriate EIMR bit is set. TLR JTAGTest-logic-reset state. Test/emulation logic is reset/disabled whilethis signal is active. This is will be the case when an emulator is notconnected or the emulator is reset. TMS JTAG Test Mode Select. Directsthe next state of the IEEE 1149.1 state machine. TRST- JTAGTest-Logic-Reset (active low). Causes all test and debug logic in a TIdevice to be reset, along with the IEEE 1149.1 interface. VISIBILITY Theability to determine what a device is doing - what it's registers andmemory contents are, what instructions it is about to execute, etc.WATCH- Analysis event which triggers when the contents of a POINT databus match that of a comparator register. In contrast, a breakpointusually refers to an address bus. XDS-524 The XDS-524 Breakpoint,Timing, and Trace Analyzer uses an C27x-E device to trace activity ofthe C27x's internal buses. XDS_RE- Debug Reset signal. Provides adedicated XDS-524 SETN reset signal

TABLE 20 Table of Nasty Boundary Conditions - Execution Control INTMState Debugger Mode Intreq INT Explain Any Any None No No int request,so no int Any Halted No step DBGIER Yes When halted, ints enabled byrequest & IER both DBGIER & IMR can be taken, even if INTM isn't set AnyHalted STEP 1 Stopmode Any No STEP 1 (ints blocked) in stopmode, so noint Enable Halted RUN 1 Stopmode IER Yes RUN 1 in stopmode, so takeenabled int Any Halted STEP 1 Realtime DBGIER Yes STEP 1 in realtime -take & IER DBGIER int and then STEP following instr Enable Halted RUN 1Realtime IER Yes RUN 1 in realtime - take IMR int to first instr of ISRDisable Halted RUN 1 Realtime Any No RUN 1 in realtime - only take intif INTM = Enable Enable Halted RUN N DBGIER Yes RUN N - does it reallymake & IER sense to require DBGIER? Disable Halted RUN N Any No RUN N -only take int if INTM = Enable Enable Running IER Yes Running - takeenabled ints Disable Running Any No RUN N - only take int if INTM =Enable Enable In ISR IER Yes If interrupts are enabled inside an ISR,can take more ints Disable In ISR Any No Only take int if INTM = Enable

Table 21 lists memory mapped registers within processor core 15 (seeFIG. 1) that are relevant to an understanding of this embodiment of thepresent invention. Processor core 15 also has a number of other memorymapped registers that are used for various control, status, andoperational tasks. Table 22 describes bits within a status/controlregister ST1 of core processor 15 that are relevant to an understandingof the present invention, while Table 23 summarizes the bit assignmentsof status/control register ST3.

TABLE 21 Memory Mapped CPU Registers MMR Word Register Bit RegisterAddress (Hex) Description Field ST1 07 System control register [15-00]ST3 37 System control register [15-00]

Emulation circuitry 851 implements the state machine described withreference to FIG. 3-FIG. 6. Interrupt circuitry according to FIG. 8-FIG.14 is included with processor 100.

TABLE 22 Status/Control Register ST1 ABORTI bit 13: Emulation controlABORTI = 1 Indicates that an interrupt service routine (ISR) is not bereturned from. This signal is exported to an emulation support module.This clears the IDS (interrupt during debug) and HPI (high priorityinterrupt) bits in the debug status register and resets the Debug FrameCounter. This causes the emulation software to disregard any and alloutstanding debug states entered from high priority interrupts since theprocessor was stopped by an emulation event. ABORTI = 0  Defaultoperating mode ABORTI is cleared at reset. EALLOW bit 14: Emulationaccess enable bit EALLOW = 1 Non CPU emulation registers write accessenabled. EALLOW = 0 Non CPU emulation registers write access disabledEALLOW bit is cleared at reset. The current state of EALLOW isautomatically saved during an interrupt/trap operation. The EALLOW bitis automatically cleared by the interrupt or trap. At the very start ofan interrupt service routine (ISR), access to the non-CPU emulationregisters is disabled. The user can re-enable access using theinstruction: bit(ST1,EALLOW) = #1. The [d]re- turn_int instructionrestores the previous state of the EALLOW bit saved on the stack. Theemulation module can override the EALLOW bit (clear only). The clearfrom The emulation module can occur on any pipeline slot. In case ofconflict the emulator access get the highest priority. The CPU has thevisibility on emulator override from EALLOW bit read. DBGM bit 15: Debugenable mask bit DBGM = 1 Blocks debug events from time critical portionsof the code execution. Debug access is disabled. DBGM = 0 Debug accessis enabled. The current state of DBGM is automatically saved during aninterrupt/trap operation. The DBGM bit is automatically set by theinterrupt or trap. At the very start of an interrupt service routine(ISR), the debug events are blocked. The user can re-enable debug accessusing the instruction bit(ST1,DBGM) = #0. The [d]return_int instructionrestores the previous state of the DBGM bit saved on the stack. Thepipeline protection scheme requires that DBGM can be set/clear only bythe dedicated instruction bit(ST1,k4) = #1, bit(ST1,k4) = #0. ST1 accessas memory mapped register or bit(Smem,k4) = #0, bit(Smem,k4) = #1,cbit(Smem,k4) have no effect on DBGM status bit. Emulation has R/Waccess to DBGM through DT-DMA. DBGM is set at reset. DBGM is ignored inSTOP mode emulation from software policy. estop_0( ) and estop 1( )instructions will cause the device to halt regardless of DBGM state.INTM Global Interrupt Enable

TABLE 23 Status/Control Register ST3 HOMP Bit 0: Host only access modeto Peripherals HOMR Bit 1: Shared access mode to HPI RAM 802 HOMX Bit 2:Host only access mode: HOMY Bit 3: Host only access mode: This bitoperates the same as HOMX. HINT Bit 4: Host interrupt XF Bit 5: ExternalFlag: CBERR Bit 6: CPU bus error: MP/NMC Bit 11:Microprocessor/microcomputer mode: AVIS Bit 12: Address visibility modeCACLR Bit 13: Cache clear CAEN Bit 14: Cache enable CAFRZ Bit 15: Cachefreeze ST3[10:7] Unused status register bits

FIG. 18 is a schematic representation of an integrated circuitincorporating processor 100. As shown, the integrated circuit includes aplurality of contacts for surface mounting. However, the integratedcircuit could include other configurations, for example a plurality ofpins on a lower surface of the circuit for mounting in a zero insertionforce socket, or indeed any other suitable configuration.

FIG. 19 illustrates an exemplary implementation of an example of such anintegrated circuit in a mobile telecommunications device, such as amobile telephone with integrated keyboard 12 and display 14. As shown inFIG. 19, the digital system 10 with processor 100 is connected to thekeyboard 12, where appropriate via a keyboard adapter (not shown), tothe display 14, where appropriate via a display adapter (not shown) andto radio frequency (RF) circuitry 16. The RF circuitry 16 is connectedto an aerial 18.

Fabrication of data processing device 10 involves multiple steps ofimplanting various amounts of impurities into a semiconductor substrateand diffusing the impurities to selected depths within the substrate toform transistor devices. Masks are formed to control the placement ofthe impurities. Multiple layers of conductive material and insulativematerial are deposited and etched to interconnect the various devices.These steps are performed in a clean room environment.

A significant portion of the cost of producing the data processingdevice involves testing. While in wafer form, individual devices arebiased to an operational state and probe tested for basic operationalfunctionality. The wafer is then separated into individual dice whichmay be sold as bare die or packaged. After packaging, finished parts arebiased into an operational state and tested for operationalfunctionality.

An alternative embodiment of the novel aspects of the present inventionmay include other circuitries which are combined with the circuitriesdisclosed herein in order to reduce the total gate count of the combinedfunctions. Since those skilled in the art are aware of techniques forgate minimization, the details of such an embodiment will not bedescribed herein.

Thus, there has been described emulation and debug circuitry that can beincorporated into a variety of digital systems. A stop mode of operationis provided in which an associated processor stops processinginstructions in response to a debug event. A real-time mode of operationis provided in which the processor stops processing backgroundinstructions in response to a debug event, but in which high priorityinterrupts are still processed. Interrupts are classified and processedaccordingly when the processor is stopped by a debug event. Whilesuspended for a debug event, a frame counter keeps track of interruptdebug state if multiple interrupts occur. While running or suspended,the emulation circuitry can jam an instruction into the instructionregister of the processor to cause processor resources to be read orwritten on behalf of the emulation circuitry. Read/write transactionsare qualified by an expected frame count to maintain correspondencebetween test host software and multiple debug/interrupt events.

In accordance with another aspect of the present invention, the machinestate can be read or written qualified by an expected suspend state.

In accordance with another aspect of the invention, a processor providesthe debug interface access to system resources (registers and memory)where access to the processor resources can be qualified by one or morelockout conditions.

The processor has access to the processor resources qualified by aprogram generated lockout, managed with the execution of instructions.

The processor has access to the processor resources qualified by aninterrupt generated lockout, which is managed with the servicing ofinterrupts.

The processor has access to the processor resources qualified by thenumber of debug events that have not been completely processed.

The processor has access to the processor resources permitted when theexecution of code is suspended.

The processor is connected to a test host where the debug software candirect the processor to use or ignore any combination of the lockoutqualifiers

The processor has circuitry to save the lockout qualifiers wheninterrupt services begin and are restored when interrupt services end.

As used herein, the terms “applied,” “connected,” and “connection” meanelectrically connected, including where additional elements may be inthe electrical connection path.

While the invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various other embodiments of the invention will beapparent to persons skilled in the art upon reference to thisdescription. It is therefore contemplated that the appended claims willcover any such modifications of the embodiments as fall within the truescope and spirit of the invention.

What is claimed is:
 1. A digital system comprising a processor, whereinthe processor is operable to execute a sequence of instructions obtainedfrom an instruction bus connected to an instruction memory circuit,wherein the processor comprises: system resources including registersand at least one memory circuit; condition circuitry for storing atleast one condition; test port circuitry for receiving test commandsfrom a remote test host; emulation circuitry for debug events connectedto the test port circuitry, operable to cause the processor to enter adebug suspend state in response to a debug event and to leave the debugsuspend state to resume execution of the sequence of instruction inresponse to a command received by the test port circuitry, whereinexecution of the sequence of instructions ceases while the processor isin the debug suspend state; wherein the emulation circuitry is operableto access the system resources in response to an access command receivedby the test port circuitry if a first condition of the at least onecondition is in a first state; and wherein the emulation circuitry isblocked from entering the debug state if the first condition is in asecond state.
 2. The digital system of claim 1, wherein the conditioncircuitry comprises debug enable mask bit (DBGM) circuitry operable tostore either the first state or the second state in response to aninstruction executed by the processor from the sequence of instructions.3. The digital system of claim 1, further comprising: interruptcircuitry for receiving a plurality of interrupt signals, operable tointerrupt execution of the sequence of instructions, wherein theprocessor is operable to cease execution of the sequence of instructionsin response to a first interrupt received by the interrupt circuitry andto execute a corresponding first interrupt service routine (ISR); andwherein the condition circuitry comprises high priority interrupt (HPI)circuitry operable to store the second state in the first condition ofthe condition circuitry in response to the first interrupt and furtheroperable to store the first state in the first condition of thecondition circuitry at the completion of the first ISR, whereby debugevents are blocked during execution of the first ISR.
 4. The digitalsystem of claim 3, wherein the processor is operable to save at least afirst condition of the at least one condition as part of a firstprocessor state during an interrupt context switch in response aninterrupt signal; and wherein the processor is further operable torestore the at least first condition during an interrupt return contextswitch.
 5. The digital system of claim 1, wherein the emulationcircuitry comprises: a memory unit coupled for accessing the systemresources in response to an access command received by the test portcircuitry; memory unit cycle type (MCT) circuitry connected to thememory unit operable to specify a selected access type to be performedby the memory unit; and wherein the emulation circuitry is operable toaccess the system resources in response to an access command for theselected access type received by the test port circuitry even if thefirst condition of the at least one condition is in the second state,whereby a rude access type is performed by the emulation circuitrywithout regard to the condition circuitry.
 6. The digital system ofclaim 1 being a cellular telephone, further comprising: an integratedkeyboard connected to the processor via a keyboard adapter; a displayconnected to the processor via a display adapter; radio frequency (RF)circuitry connected to the processor; and an aerial connected to the RFcircuitry.
 7. A digital system comprising a processor, wherein theprocessor is operable to execute a sequence of instructions obtainedfrom an instruction bus connected to an instruction memory circuit,wherein the processor comprises: system resources including registersand at least one memory circuit; condition circuitry for storing atleast one condition; test port circuitry for receiving test commandsfrom a remote test host; emulation circuitry for debug events connectedto the test port circuitry, operable to cause the processor to enter adebug suspend state in response to a debug event and to leave the debugsuspend state to resume execution of the sequence of instruction inresponse to a command received by the test port circuitry, whereinexecution of the sequence of instructions ceases while the processor isin the debug suspend state; debug frame counter (DFC) circuitry operableto store a debug frame number corresponding to a first debug event,wherein the test port circuitry is operable to communicate the framecount number to the external test host; debug frame register (DFR)circuitry, operable to store an expected frame count number in responseto a command received by the test port circuitry; and wherein a firstaccess command received by the test port circuitry is not performed ifthe frame count number is different from the expected frame countnumber.
 8. A method of operating a digital system comprising a processorwith system resources including registers and at least one memorycircuit, comprising the steps of: executing a sequence of instructionsrepresentative of a program; setting a debug mode within the digitalsystem to one of a plurality of states; generating a debug event inresponse to executing the sequence of instructions; and entering into adebug suspend state in which execution of the sequence of instructionsis suspended in response to the debug event if the debug mode is in afirst state, but blocking entry into the debug state if the debug modeis in a second state.
 9. The method of claim 8, further comprising thesteps of: interrupting execution of the sequence of instructions inresponse to a certain interrupt event, and executing a correspondinginterrupt service routine (ISR); and setting the debug mode to thesecond state in response to the certain interrupt event, and thensetting the debug mode to the first state at the completion of the ISR,whereby debug events are blocked during execution of the ISR.
 10. Themethod of claim 8, further comprising the steps of: generating a uniqueframe count number corresponding to a first debug event, andcommunicating the frame count number to the external test host; storingan expected frame count number in response to a command received by thetest port circuitry; and receiving an access command that includes aframe count number from the remote test host, wherein the access commandis not performed if the received frame count number is different fromthe expected frame count number.
 11. The method of claim 8, furthercomprising the steps of: saving the state of the debug mode as part of afirst processor state during an interrupt context switch in response aninterrupt event; and restoring the state of the debug mode during aninterrupt return context switch.